Revit tutorials - create detailed curtain wall mullion 

Well, for the predefined Revit corner mullion types you cannot do that.

just make a corner family, and load into it the chosen CAD file from the manufacturer. Adding the CAD file means that you've picked an actual product, not a generic profile. So for the design stage you might wait, then when the specification part comes out, you update the mullions.

so far 3D one can be only single closed loop, but in the roadmap they have the request for multiple loops for mullion profiles, so maybe in next release it will be posssible ;)

@@eMexperience amazing