There was a 9mm conversion for the AUGs but they still used a propriety magazine. I have never heard of a conversion for one to an MP5 magazine and if by some miracle someone made a conversion magwell for an airsoft rifle I would imagine it would be to run replica 9mm AUG magazines and not MP5 ones. I dont think it would work anyway though as the feed nipple sits towards the front of the magwell but the magazine release is at the back. A MP5 magazine is a lot shorter so the magazine catch would need a hell of a lot of work. To actually do it it would cost more then just buying some regular AUG magazines.
Edit
Here is a link to some info on the real steel ones: