Perhaps, after many sessions, if you two hit it off and are chatting, then you could express the desire to go "a little further"  and see if that piques her interest.

If she fails to respond or brushes it off, then don't bring it up again.

FBSM gals are fully aware that men often would like to go further, but they have made a conscious decision to stay within certain boundaries, and that should be respected.

By the way, I have met many massage only providers and have gone to FS with some of them as a result of their invitation.  So remember that often good things come to they who wait.

In the meantime, enjoy the massage.

I have been seeing a provider who only does body rubs.  She does them topless and is not FS.  I really would like her to be totally nude and would love a chance to DATY even if it's not FS for me.
How do I bring that up?

I have one Fav...  FBSM provider...  we long ago crossed that line & we do FS & pretty much everything.  My donation is comparable to ladies who advertize FS.

I have another Fav...  FBSM provider...  she goes totally nude.  Full touching is allowed but not much on finger penitration.  I do enjoy taking her breasts in my mouth & tounge playing them until she cumms...  all while she is trying to get me off.   She's not encouraging DATY or FS...  or BJs...   Her rate is very reasonable & sometimes I just feel like being the center of attention.  

On occasion of a heated encouter, I have touched an area & asked "May I?"  Other times, one must allow the lady to guide by her movements.  I think if you have met her several times & are comfortable with each other...  you might ask "I'd love to kiss you all over."  See what her response is.  If she's keeping her panties on, she's not really encouraging going there.  
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If you push her boundaries & she's feeling pressured to do things she does not want to...  expect to be refused appointments in the future.  See it from her side...  clients who are insistant on services she does not offer, make her feel threatened & even afraid.  You can't hope to then get good service.
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Respect a woman's limits. If she has not allowed more by now, she will not be providing more. Move on to someone that does.