I don't know if there is a way to convert old sim files to US2. Universe Sandbox 2 is a rewritten, overhauled, and reworked simulator. If they did have the ability to convert old sim files, it may not work properly. US2 has new physics, gravity, and calculations to simulate orbits and collisions, and that's something the old game didn't have.
A file converter is always possible I guess, but if Dan and the team wanted to further increase the complexity of Universe Sandbox, backward compatibility with US1 might inhibit this ability.
I could be wrong though, as I am not a game developer, but that's what I would think.