Unfortunately there is no way to place a support on a support. One work around we see quite frequently is to draw pipe for the dummy leg and mark it existing, then attach the shoe to the dummy leg.
I hope this makes sense,
we do what Matt is talking about often except we leave as new and change the long description to read your Dummy Leg callout and have the shoe call out in the BOM. You will have to tap the pipe with the center of ell for it to drop out.
How can the Quantity for the dummy leg be changed to 1 instead of the "length of pipe" in this example?