Yes—this is generally feasible.

Canon EOS film and digital bodies use the same EF mount registration distance, so there’s no inherent optical reason the lens would focus differently just because you moved it between those two cameras. In practice, tiny manufacturing tolerances exist, but they’re usually very small.

For star trails, this approach should be good enough, especially since film flatness and other real-world variables can introduce as much or more variation than the body swap itself. One caution: with STM lenses, focus changes in small electronic steps, and handling the lens during the swap could shift it slightly.

Also note that the hard stop at the far end of the focus ring is often past true infinity, so simply turning it all the way may not give sharp stars. Using live view on the digital body to confirm focus first is the better method.

So yes: focus on the T6 in live view, then carefully mount the lens on the 3000N without touching focus. It should work.