The GMG has a fan in the hopper for blow back. Can I put a hopper extension on it with a lid?  I just dont want to create a hopper fire. 

Seems like some folks on the old site did.  It needs to be a good fit and have the lid, if I remember correctly.  I never had an issue with mine running out of pellets.  I've done overnight cooks at low temp with no problem.

 
 I agree with the"good fit"- above. The purpose of the hopper fan is to generate enough air flow to pressurize the auger assembly. If the extension allows for more leakage than the fan is capable of

 providing, there could be a problem with burn back.. The fan has to generate enough flow to offset leakage PLUS enough to pressurize the auger assembly.

 These pits can run a long time w/o replenishing pellets unless you are cooking at max temps. Then someone would be there to to R/R the food.. Just curious as to why you are considering a hopper extension.

I did a pork butt and it seemed to get low on pellets after about 6 hours.  I did start it off at 350 for an hour and maybe not full to the top.  I just thought with an extension there would be not worries at all.