First, go to my site and download the object id inserter program. Use it to insert 1 object after the object ID of the last already existing attachment in your model. In the newly modified MDL, scroll down to after the last attachment and duplicate it. Change the object id to 1 more than it was and change the name to how you want it (For example, "Foot Left Ref", for the left foot). Next, go to the PivotPoints sections and find the one which has been marked with its ObjectId in a comment. Change that PivotPoint to be where you want the new attachment.
If you need more detailed instructions for any of those steps, I can try to help more.