Interesting you say this. Neither my wife nor her (and now my) trainer can do a pullup. Trainer maybe... So she does, and has my wife do, "jumping pullups". My father also (who is 79, BTW) when she trains him. She recently had me doing a sequence of 5 deadlift reps followed by burpees, I think, 10 reps or maybe 15, followed by pullups. I forget how many reps, probably 10. When I start my back workout with pullups, I can usually get a set of 12 or so, then 10, then maybe 8 or something like that before I can't get myself up to the bar. Doing this sequence starting with dl, I was pretty fatigued by the time I got to the pullups. She had me doing these jumping pullups when I hit the failure point. I thouggt it was ridiculous, but if I just give myself a boost enough to get moving, my lats still do the upper part of the movement plus I get the negative. So it does have a benefit. And according to Soonya (trainer), this is a way to build the strength required to do a pullup.
I always figured lat pulldowns would be a good way to work up to pullups, but maybe not? I woukd think if you sat straight at the lat pd machine, and use only lats, shoulders and arms, mimicing the movement of dead hang pullups, it would be very similar...