I've created a rather complex animation - 180 frames, more than 2/3 of them are keyframes.

Unfortunately, in retrospect, it's a bit too jerky. The simplest way to make an animation smoother, is to add more normal frames in between the keyframes. But with so many of them, I'm looking at a lot of work.

Is there anything like a script to automate this task? Maybe adding extra non-keyed frames to the spaces between keyframes, depending on how many are already there?

Try my Smoother script:

http://ockhamsbungalow.com/Python/Smoother3.zip

It says that "deletes glitchy keyframes" . That's not at all what I'm looking for :/

All of the keyframes are necessary, I don't want to delete any. It contains a lot of precise movements. I just want to add more interpolated frames between them. I'm well aware this will make the animation both longer and slower, that's the intention.

If you want to slow the whole thing down, Poser's internal retiming feature
would do that job, I'd think?

great. Where is it, how do I use it, etc.. ?

Go to the top bar, Animation, then choose the retime animation option. 

Have you tried the skip frames option?