user categories variables as array ?

hello

question :
the user categories variables are stored as individual
is it possible to store them as an array or object ?

explanation :
actually, my $user looks like this :

stdClass Object
(
    [uid] => 76
    [name] => aa
    [pass] => 4124bc0a9335c27f086f24ba207a4912
    [mail] => aa@aa.aa
    [mode] => 0
    [sort] => 0
    [threshold] => 0
    [theme] => 
    [signature] => 
    [signature_format] => 0
    [created] => 1265180387
    [access] => 1265239017
    [login] => 1265238406
    [status] => 1
    [timezone] => 3600
    [language] => 
    [picture] => 
    [init] => aa@aa.aa
    [data] => 
    [form_build_id] => form-a35ee745d65a45d54b6ef4f78b22a1bf
    [lastname] => aa
    [username] => aa
    [roles] => Array
        (
            [2] => authenticated user
        )
    [cat1_test_a] = test
    [cat1_test_b] = testb
    [cat1_test_c] = testc
)

i would like to use user categories to store extra informations like :

stdClass Object
(
    [uid] => 76
    [name] => aa
    [pass] => 4124bc0a9335c27f086f24ba207a4912
    [mail] => aa@aa.aa
    [mode] => 0
    [sort] => 0
    [threshold] => 0
    [theme] => 
    [signature] => 
    [signature_format] => 0
    [created] => 1265180387
    [access] => 1265239017
    [login] => 1265238406
    [status] => 1
    [timezone] => 3600
    [language] => 
    [picture] => 
    [init] => aa@aa.aa
    [data] => 
    [form_build_id] => form-a35ee745d65a45d54b6ef4f78b22a1bf
    [lastname] => aa
    [username] => aa
    [roles] => Array
        (
            [2] => authenticated user
        )
    [cat1] = array
        (
            [test_a] = testa
            [test_b] = testb
            [test_c] = testc
        )
)

i can do this forcing the values, but this is not available when using the user profile edit pages