Hey Jose,
Unfortunately, my work role has changed a little. So having solved this problem a month or two back, I was then told that we will no longer be using Moodle. So I haven't looked at this for a while now.
From memory, the important bit is that in order to be able to sort by a particular field, that field must belong to the mdl_user table in your db. I used PhpMyAdmin and was able to copy across fields from other tables. I think the table that stores custom field data is called mdl_user_info_data.
The rest was mostly cosmetic ie. adding 'GroupID' heading to the Gradebook table etc. My only experience of PHP was with moodle, so I always tried to copy code from the original files rather than create my own.
I probably won't check here again since I won't be using Moodle much anymore, but I know how frustating it can be when no one helps you out, so if you need more help contact me at philip.walley@sophos.com
Good luck