Just in the source code, the DB will be changed automatically.
Without a lot more information, I can't even begin to speculate what might be the issue.
You want to add more fields than are currently allowed; got it. That requires a source code change in the info.jtrac.domain.Field.Name enum. If you look at it, it should be relatively obvious how to change it by adding fields. (Make sure to add new fields at the end of the list, not in the middle - these are Java enums!) I don't know why it was implemented this way, but there's a comment about it at the top of the info.jtrac.domain.MetaData class.
I don't understand the problem. If you want no more than 5 additional text fields, then don't add more than 5 additional text fields, and don't give admin access to anyone else who might do so.
What stops you from adding those fields?
Custom fields for Space
Duplicate of https://sourceforge.net/p/j-trac/support-requests/33/, so I'm closing it. Don't add anything to this ticket.
Add Custom fields for Space