RISA News has posted a how-to on appending and merging multiple models into a single model file.
RISA-3D allows you to append, or merge, multiple models into a single model file. This may be helpful if you model single floors in separate files and want to stack them together.
Before appending the files together you must be sure that each file has the same Units and Global Axesdefined. Once this has been verified, open a model and go to File > Append. Then choose the model you wish to add to the currently open model.
via [RISA News]