In TFtNA they write about how Sadie Bjornsen had noodle arms and then after training she could do weighted pull-ups. I couldn’t find a forum post about it though, but this thread has advice about how to get to a single pull-up:
I can’t do a pull-up either, nor a chin-up (but I’m getting soooo close). I’m using resistance bands so I can do a full set assisted. It’s also a psychological boost to be able to do a full pull-up even if I’m being helped out by the band. I just started this routine so I can’t vouch yet that it works but I’m optimistic.
I’m pretty sure I’ve also read here about doing negatives — lowering yourself slowly from the pull-up bar after getting into pull up position in anyway you can.