You certainly can.
You can charge for all aspects of your XOOPS services - Hosting, installing, upgrading and developing for XOOPS sites are all very well suited for paid services.
You can even develop a module and charge people for downloading it. What you cannot do is set limits on what people can use your work for, once they have acquired the source code. Hence it is illegal to disallow people distributing your code after purchase.
It is also breaking the license to encode the XOOPS code and sell it is a package without also supplying the un-encrypted source code.
Legal (Have to do)
>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)
>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