Yes, there are no restrictions on what kind of services, you can offer with your XOOPS website or how you charge for them.

Some other considerations :

Legal (Have to do)
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>You may use Xoops for anything you want.
>You must leave the credits in the source code.
>You do not have to have "powered by" or any kind of display credit
>You may redistribute, even sell the code.
>You may not restrict the redistribution rights of the code
>You may change, add to, delete from, and intergrate with the code.
>You do not have to release your modifications.
>If you do release your modifications (including modules) they must be GPL or less restrictive.

Polite (Would be nice)
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>Give Xoops and the hard working developers the credit and recongition it deserves
>Release your improvements
>If you benifit from Xoops - GIVE BACK! Money, Time, Code, or Something Else.

see also this FAQ HERE

and Here