Welcome aboard. You raise a good question.
The answer will depend on your ethics. If you dig deep enough on the forum, you'll notice some male members posting about confusion by "mixed messages." The same can hold true for women as well.
If you already have a FWB type regular situation, then it's best to leave well enough alone.
A gal that is only into you as a physical release will be turned off by expressions of affection. And a gal that is secretly open to advancing the situation will end up with false hope and hurt.
Years ago, I had a regular FWB that was going along smoothly. Then.... one night after many drinks with her at a bar, I began getting affectionate (playing with her hair in my car, arm on her back while walking inside my house). I was too buzzed to actually comprehend her conversation that night, but later on, I recognized my mistake (she had accepted the limitations, but my drunken actions confused and ended up hurting her).
As with many women, they may desire more than just the physical, but they can "keep a cap on it." The moment we assume a role of something more than just a platonic fvck, this opens a new territory that we likely didn't bargain for (plus, a person with some ethics doesn't really want to intentionally play someone).