It would be useful, and save time, if the option to have an sdltm as a supported file type for importing TM's could be made available. Currently you have to export one of the sdltm's to a supported file type and then import this into your other sdltm. A few unnecessary steps. You also have to know that as another workaround to import an SDLTM you would have to use the Upgrade TM feature which is not intuitive.
So you are presented with this if you wish to import... no sdltm available:
This would simplify the process considerably and make it more obvious what you have to do.