The only way currently available is to leave the tiles where they are,
and extend the tileset only to the right and to the bottom. As long as
you only add tiles there, Tiled will manage to load existing maps
fine.
This is because when adding tiles at the bottom, the existing tile IDs
don't change. And when extending the tileset to the right, the tile
IDs change in a predictable manner, which is used by Tiled to adjust
to the new tile locations.
Eventually, if you're into reorganizing the tileset to make it
prettier overall or to clean out unused tiles without leaving gaps,
some kind of tool would need to be written, but I am not currently
aware of any such tool.
Best regards,
Bjørn