Why don't you just try exporting and importing characters using the built-in function?
main menu > Project > Export Data > Characters
This creates a character file "ExportedCharacters.xml" in the "Export" subfolder that you can use over and over again.
Import it with:
main menu > Project > Import into current project > Characters
Another way is to create an empty project with only the characters and then copy that for new projects.
Another possibility: Assign all the characters you want to transfer to an empty scene and transfer it via clipboard into the new yWriter project. This can be done with the scene context menu (right click on the scene).
Cheers,
Peter