Yes it is - the Yanks have done it but you'll have to spend some serious moolah on a gas powered (but crap) Hudson M3 to convert as well as the donor MP40.
I did a conversion using Mp-40 internals and a Hudson M3A1 Grease Gun. It was very hard to fit the internals inside but deffinatly worth it. I however would use Mp-7 internals the next time I make another one.
Check it out here:
http://ww2aa.proboards.com/index.cgi?bo ... hread=5179
TommyGunner
where can you purchase Hudson m3? I looked everywhere
sorry for Offtopic
To be honest I have no clue where to get Hudson Grease Guns anymore. I heard that they were being produced again but I have not looked to see who has them.
The conversion is not cheap, its around $250 for the Grease Gun and then $150 for the Mp-40. So its a $400 venture all said and done but does provide an all weather skirmishable M3A1 Grease Gun.
TommyGunner
Needs must for WW2 guns unfortunately!
According to their website, WGC have the Hudson Grease gun in stock. 
When I want your opinion - I'll tell you what it is!