dan:
If I understand your problem correctly, I would just open a second copy of ArchiCAD (there is a check box for this in the lower left corner of the Open dialog box) and select the items you want to add to the newer file (Find & Select... could help), copy them, and then paste them into the newer file. These items will come in at the same location and on the same layers (assuming the layers are the same between the two files). This has the advantage that you can see what is being added, I think a merge of some sort would be more difficult to manage.
HTH
David