Today i have made some progress in this question. On iPhone you can buy a iRig Mic which can do the job (and it should work on android too). But since i want to use another external microphone i have decided to make the adapter myself from some different adapters (which does the same thing as iRig Mic, but with a female 3.5 mm jack output, so i can use my wireless microphone with it)
I have ordered the parts from an online store in denmark, but you can get it all over the internet, you just need to know what you need :)
The first thing you need is a 3.5 mm male 4 pole conductor jack (TRRS) to RCA adapter. Like this one 
Then you need a RCA to either female jack or XLR adapter (either if your mic has a jack or XLR output) Like this one 
And thats all.
Then you can plug everything together and use the mic of your choice on your Android phone.
(BTW some stock camcorder apps can't use external microphones, then just use this camcorder app: https://market.android.com/details?id=rubberbigpepper.lgCamera&feature=search_result#?t=W251bGwsMSwxLDEsInJ1YmJlcmJpZ3BlcHBlci5sZ0NhbWVyYSJd )
I'm using this app ^^ with this set of wireless microphones 
And i'm just waiting for the last adapter in the mail tomorrow..
So from tomorrow i can record with my galaxy nexus and record the sound via the wireless microphone on a distance up too 30 meters :D
And if i need to go around with the phone and the microphone, i will just plug in a wired microphone to the female jack and go around with the phone and microphone in my hand :)
I hope someone in the future can use this information which was very hard to find on the internet :) When i have some time later today or tomorrow, i will upload a couple pictures to illustrate my setup :)