Is it okay to ask to "waive" the stipend when offered a fellowship for PhD program?
You can certainly ask for the stipend to be withdrawn. Most universities are happy to spend less money. They may not be happy if this also means that you are not doing the TA/RA work, depending on whether they have enough TAs/RAs. 

You can also ask for the stipend to apply to tuition - no harm in asking. Less likely that this will be granted than just stipend withdrawal. 

Volunteer work is usually accepted, so if you want to do the TA/RA work you may be able to, even without the stipend. The legal status of such work is the question, which you should ask a lawyer about.