Try this:
1. Open a new part by selecting your part template file.
2. Change the mass property for this part file. You do not have to have any geometry.
3. Save this as your new part template by using "Save As" and selecting part template. Make sure you are saving to the same location where your part template is already stored. The file should be in the SolidWorks directory (I point all of our installations to a network location) and named SolidWorks\data\Templates\Part.prtdot.
