Yes, I see. So if I can ally the ones that will not join anyway I can win by siegnig the ones ones that do join. Hmmm ... that is tricky in my case. They all hate me because of AE. I'll have to work on this.
if they are in coalition, declare on ally of one nation in coalition, and mak coalition ally full co-belligerent. This will drag all coalition members in, and you can do seperate peaces.
if electors are not in coalition, look if ANY elector is allied to a nation outside HRE. declare like that on all possible. then figure out a wartarget within HRE with the right allies, that might drag all remaining electors in, when you make said 'right' allies co-belligerent.
there is always a way to get all electors and emperor. It might need two-three wars at once though, if you are unlucky.
coalitioning entire HRE and then doing option one is generally easiest if you are big lategame blob. because option 2 requires studying alliance patterns like mad
