How to assign different forcefield simultaneously in sorption module

Dear Accelrys Community:

I have this problem with Sorption module in material studio.

I am calculating CH4 sorption isotherms on a metal-organic framework system (MOFs).

I use united atom model for CH4 molecule and all atom model for MOF but the MOF model contains Cu atom so it

can't be described by dreiding forcefield since it doesn't have forcefield parameters for Cu atom.

So I need to assign different forcefields at the same time but i encountered a problem when I assigned dreiding forcefield

for CH4 molecule and Compass forcefield for MOF model.

The results came out : "ERROR: The model contains forcefield types that are not part of the selected forcefield."

I need your expert help to solve this problem.

Thanks

Ken