Sounds like you want to import a PTG file as a Tracing Image so that it's overlayed on to the main image. At the moment you can't do that with the PTG format, that will be fixed in a future release, so to do this:
Originally Posted by Tom W
1. Open the original PTG file that you want to trace.
2. Use the 'Export as Image' option in the File menu to export a copy of it as a JPEG or PNG file to disk.
3. Open the template you have for your new file.
4. Select the Tools / Tracing Options / Load New Tracing Image option from the menu bar.
5. That will prompt you to load an image file, select the exported copy of your original PTG file.
The same goes with the Import Image to Layer option in the File Menu: You can import an image to its own layer but currently you can't do that for PTG file, again that will be fixed in a future revision.
The Import Image option will replace the existing file with the one you're importing, which is why you can't use it to import your old file to a layer in the new one.