Importing NASTRAN FEM and combining with .gda FEM

I imported a NASTRAN bar (beam) model into SimLab, as a *.dat model with a dozen bar element bodies therein.  

I wanted to connect this beam element model with a FEM within an assembly.gda, but the *.dat cannot be dragged directly into assembly.gda

However, if I create a submodel in assembly.gda, the *.dat model can be dragged into the submodel.

Although the beam model and FEM's are all within the assembly.gda, they cannot be connected by RBE or Join, with Warning they are not in the same model.   Apparently, there is not way to connect a NASTRAN model with FEM's within an assembly.gda.