Sorting by Add on field

Hello,

I want to sort the return of a list alphabetically by users' names. The problem is the API fetch is from the signup table (which doesn't have user_name) which has an addon for the profile table (where user_name is stored).

When I try to sort the list in the output, the only options to sort by are from the signup table (email, signup_id, created_at etc).

How can I use the user_name from the addon to sort alphabetically?

Thanks,

Gavin