The following procedure can be used to merge two models, say model A and model B:

1. Export a text file of model A via "File > Export > Text". Make sure to export all input tables, load patterns and load cases.
2. Open model B and import the previously exported text file of model A via "File > Import > Text". Select "Add to existing model" for Import Type and use the "Advanced Options" button to specify how the import should treat items with duplicate names, ie. items that already exist in the model.