Not sure how much the Galactic Empires discussion can affect a possible Spelljammer campaign dealing with Blackmoor? Though I suppose the races from that campaign could be used to populate the planets of Blackmoor's solar system. If they have been visiting Blackmoor, they could easily have visited the other planets as well?
You always need to tweak things to get crossover settings to work.
My personal thing with Spelljammer is that you must
have crystal spheres and the phlogistion for it to be the Spelljammer universe (and not another space fantasy universe). However, I see the crystal spheres as "reality barriers" that allow a GM to handwave the entire contents of an individual crystal sphere as "following the will of the local gods".
To add Blackmoor, I would only have to add a "Blackmoorspace crystal sphere" and would then look to make a set of planets that had similar playstyles to Dave Arneson's work.
If I wanted to go with the entire Galactic Empire, I would say that goes beyond the scope of a single crystal sphere. If there is a map of the Galactic Empire I would be tempted to rule that was a map of the crystal spheres in the local area around the Blackmoorspace crystal sphere and retcon all "hyperspace-style" ships into ships that travel via the phlogiston. Details of star systems within the Galactic Empire could then be used to start creating crystal spheres around Blackmoorspace.
(I don't think that other people "need" to do it the same way as me, but things wouldn't seem like Spelljammer to me if the universe was done differently. So I would try to give the Galactic Empire a Spelljammeresk backstory.)
Looking at the D20 Blackmoor material, there was the story of the Gnome who invented a machine that allowed him to travel to the Sun. Not sure if he is around anymore, assuming he succeeded?
What book is that in?