Re: John Bonham
I'm with our man TheDuke. It is entirely possible to imitate Bonham. Here's how you do it:
1) Transcribe enough that you understand his phrasing and approach to orchestration of grooves and fills.
2) Sit down with a sequencer and, by means of looping and programming sequenced drums over the top, work out exactly where Bonham is placing his notes relative to the beat.
3) Work on your tuning and technique. Tune open, hit the drums nice and hard (but without hitting "through the drum") in the centre of the head and use the right drum sizes. Spend time just hitting drums trying to work on your touch, ignoring any issues of what parts you're going to play.
If you understand those three attributes of Bonham's playing - his touch/tone, his timing and his part selection - then you're going to be able to imitate it given enough practice, recording and analysis of your own playing.
Elvin is considerably more tricky because, as TheDuke86 said, his ability to bend time is as big a part of his feel as his tone or part selection. It's not hard to sit down and work out what Elvin is playing, or how his drums sound. It's virtually impossible to work out a logical "what-would-Elvin-do" formula for his timing, which is what makes him so hard to imitate.