Add "Cut-Paste" from one .dn file to another.
The fact that I have to recreate any unique objects I build rather than simply copying them into new projects costs DAYS of time. It is a crippling deficiency in Dimension and it's shocking this is not given the highest possible priority since it is a fundamental feature in all other Adobe products. PLEASE fix this.
I understand your frustration not being able to copy and paste between Dimension files as we do not currently support multiple documents open at the same time. We will fix this in the future.
For now your best bet might be to export the geometry you want to use for multiple projects via File>Export>Selected Models...