Pretty much Phasma was a high ranking stormtrooper soldier or whatever. the Rebels/Resistance could have offered her to join them and ask her to help them in finding and training more soldiers if possible.
But...why would they do that? Why would she do that? I'm assuming you're implying she'd get some sort of deal for immunity? I just don't understand why you'd come to this thought. I can't see any aspect of her character actively choosing to go AWOL and join up with the Resistance, nor the Resistance ever offering her that option. They got what they needed out of her without having to "recruit" her, which even in and of itself offering her a position in the Resistance opens yourself up to a potential betrayal from her.
The First Order gave her everything she wanted, and even allowed her to do things her way in some cases. This resulted in her betraying both her brother and their clan, along with other things was willing to do in order to get what she wanted. Stuff that she would be required to undo if she did accept the idea that rebuilding the New Republic is more important than everything she did gain in the First Order.