You can plug them into the mic jack on the D5100 but if you're going to this much trouble you may want to add a dedicated audio recorder like the Roland R-05 which I use or a Zoom H4n which has the advantage of 4 track recording.
Oh - I should add, this question might draw more responses and replies from others far more qualified, posted in the DSLR forum, in fact the moderators may even move it there as the question concerns DSLR video technique that's not specific to any camera model.
#2. "RE: Wireless Microphone Question." In response to Reply # 0 Mon 03-Oct-11 04:53 PM by gkaiseril
If you are going to mike multiple individuals, I would also look at getting a portable mixer. I would also be looking at low impedance balanced lines as they reduce a lot of electrical interference or inductive noise.