I think the boat has passed on an Alien Nation
continuation, sadly. The series ended in 1990, and the last TV movie (a ratings failure) aired in mid-1997. The cast and the crew has aged quite a bit, and it's not a property with more than limited nostalgia value.
It's such an interesting and perpetually relevant concept (perhaps more so now than in 1988-1997) that I hope it's returned to eventually, though. A shame Tim Minear's remake didn't come to fruition.