want to add file upload field to core Profile module in Drupal6??

This would be a wonderful feature, though I'm not sure it will be added to the profile core. I will take a few hacks at it now, post progress..

you may change the user.module and insert the form upload file
if (!$register){
$form['#attributes']['enctype'] = 'multipart/form-data';
$form['document_upload'] = array(
'#type' => 'file',
'#title' => t('File'),
'#required' => FALSE,
'#description' => t('Click "Browse" to select a file to upload. The maximum file size per upload is %size MB.', array('%size' => round(file_upload_max_size()/(100000*100000), 0))),
'#weight' => -5,
);
$form['#validate'] = array('user_node_form_validate');
}

I added the if statement to check the user uid since it will be difficult to get the uid in the process of registration
then added the following function

function user_node_form_validate($form, &$form_state) {
if (isset($form['document_upload'])) {
$validators = array(
'file_validate_extensions' => array('txt')
);
print_r($forms);
$file = user_save_upload('document_upload', $validators);
if (!$file)
form_set_error('upload', 'You must select a valid file to upload.');
else {
$form_state['values']['title'] = $file->filename;
$form_state['values']['myfile'] = $file;
}
}
}

function user_save_upload($source, $validators = array(), $dest = FALSE, $replace = FILE_EXISTS_RENAME) {

global $user;
static $upload_cache;

// Add in our check of the the file name length.
$validators['file_validate_name_length'] = array();

// Return cached objects without processing since the file will have
// already been processed and the paths in _FILES will be invalid.
if (isset($upload_cache[$source])) {
return $upload_cache[$source];
}

// If a file was uploaded, process it.
if (isset($_FILES['files']) && $_FILES['files']['name'][$source] && is_uploaded_file($_FILES['files']['tmp_name'][$source])) {
// Check for file upload errors and return FALSE if a
// lower level system error occurred.
switch ($_FILES['files']['error'][$source]) {
// @see http://php.net/manual/en/features.file-upload.errors.php
case UPLOAD_ERR_OK:
break;

case UPLOAD_ERR_INI_SIZE:
case UPLOAD_ERR_FORM_SIZE:
drupal_set_message(t('The file %file could not be saved, because it exceeds %maxsize, the maximum allowed size for uploads.', array('%file' => $source, '%maxsize' => format_size(file_upload_max_size()))), 'error');
return 0;

case UPLOAD_ERR_PARTIAL:
case UPLOAD_ERR_NO_FILE:
drupal_set_message(t('The file %file could not be saved, because the upload did not complete.', array('%file' => $source)), 'error');
return 0;

// Unknown error
default:
drupal_set_message(t('The file %file could not be saved. An unknown error has occurred.', array('%file' => $source)), 'error');
return 0;
}

// Build the list of non-munged extensions.
// @todo: this should not be here. we need to figure out the right place.
$extensions = '';
foreach ($user->roles as $rid => $name) {
$extensions .= ' '. variable_get("upload_extensions_$rid",
variable_get('upload_extensions_default', 'jpg jpeg gif png txt html doc xls pdf ppt pps odt ods odp'));
}

// Begin building file object.
$file = new stdClass();
$file->filename = file_munge_filename(trim(basename($_FILES['files']['name'][$source]), '.'), $extensions);
$file->filepath = $_FILES['files']['tmp_name'][$source];
$file->filemime = file_get_mimetype($file->filename);

// Rename potentially executable files, to help prevent exploits.
if (preg_match('/\.(php|pl|py|cgi|asp|js)$/i', $file->filename) && (substr($file->filename, -4) != '.txt')) {
$file->filemime = 'text/plain';
$file->filepath .= '.txt';
$file->filename .= '.txt';
}

// If the destination is not provided, or is not writable, then use the
// temporary directory.
if (empty($dest) || file_check_path($dest) === FALSE) {
$dest = file_directory_temp();
}

$file->source = $source;
$file->destination = file_destination(file_create_path($dest .'/'. $file->filename), $replace);
$file->filesize = $_FILES['files']['size'][$source];

// Call the validation functions.
$errors = array();
foreach ($validators as $function => $args) {
array_unshift($args, $file);
$errors = array_merge($errors, call_user_func_array($function, $args));
}

// Check for validation errors.
if (!empty($errors)) {
$message = t('The selected file %name could not be uploaded.', array('%name' => $file->filename));
if (count($errors) > 1) {
$message .= '

• '. implode('
• ', $errors) .'

';
}
else {
$message .= ' '. array_pop($errors);
}
form_set_error($source, $message);
return 0;
}

// Move uploaded files from PHP's upload_tmp_dir to Drupal's temporary directory.
// This overcomes open_basedir restrictions for future file operations.
$file->filepath = $file->destination;
if (!move_uploaded_file($_FILES['files']['tmp_name'][$source], $file->filepath)) {
form_set_error($source, t('File upload error. Could not move uploaded file.'));
watchdog('file', 'Upload error. Could not move uploaded file %file to destination %destination.', array('%file' => $file->filename, '%destination' => $file->filepath));
return 0;
}

// If we made it this far it's safe to record this file in the database.

//print_r($user);
//print_r($user->uid);

$file->uid = $user->uid;
$file->status = FILE_STATUS_TEMPORARY;
$file->timestamp = time();
drupal_write_record('files', $file);

// Add file to the cache.
$upload_cache[$source] = $file;
return $file;
}

return 0;
}

this will upload to the temp file depending on where you point the files the rest is up to you to use copy files. I try to look for an existing module but there are no available so far for drupal 6

cheers