An Addon module for ACL

Hi.

I would like to give my users access to the site after I have verified that there debit order has been successfully debited. I use a desktop application to tell me what users have payed and what users did not pay.

The list file my desktop application produces is a xml file. I can then FTP this file to my site folder and now only need a module to read the XML user list and block or un-block my users.

I know that your module is the API, and I cant write my own module. Where would I begin?

Thanks