Possible workaround?

You kill her in camp (if attacking her would cause her to turn hostile you can for example create a surface and make her walk in it). Once dead you give her soul to Withers to look after so durge can't interact with her, then long rest and resurrect her the next morning and she will be a permanent party member (without any banter, reactions or story, so pretty much the same as a hireling, but, it's Alfira).