Have you ever watched a television show or a movie and saw something you really wanted to incorporate into one of your games but didn't because it wasn't Star Wars? I'm sure almost everyone has. However, why not throw it into your games?
Did you like that new alien species in the last episode of Star Trek or like the starfighters in that bad movie Wing Commander? Often times, other sources are some of the best to utilize for new gaming material. Just keep saying to yourself, "the Star Wars Universe is immense."
I often times throw in things from movies, television shows and books into my games. Not only is it fun, but it really keeps the players on their toes. I've gone as far as to incorporate a nasty vampire for a new villian, based highly upon the master vampire in the movie John Carpenter's Vampires. The players loved it, even though they were a bit unsure of the whole ordeal at first.
Now, how about some helpful advice to aide you in crossing over new items into your games? Here's the guidelines I have personally developed and used time and time again.
This should be the easiest thing to convert to the Star Wars Universe. Star Wars spans such a large setting that it could be quite possible that a species exists which very closely matches Romulans, Vorlons, Drej, Protoss, Spathi, etc. Make up race stats to the best of your abilities with what you know about the species, and what you don't know, do your best to guess. You don't have to get it 100% correct.
So you liked Braveheart a lot? Want your characters to meet William Wallace? Well, as you should be saying, William Wallace doesn't seem to fit into the Star Wars Universe at all, and right you are! However, William Wallace was a good character - referring only to the theatrical pertrayal and not the real historical figure - and can be used as a basis for a similar non player character. Keep in mind that the easiest types of characters to convert over are characters from a science fiction setting, many can directly be brought over with little or no modification.
Redesign and write up the character carefully. Be sure to incorporate this character's distinctive traits and habits. Also be sure to have a plan of how you intend to use this character in your games, be it either a chance one time encounter or a prolonged battle between him and your players.
Introduce this new character in one way or another to the rest of your group. He may be a new helpful friend, or he may be the new hated enemy. Find a way for your players to learn of him, either by word of mouth or a direct meeting. Be sure to use your best game master skills to make this initial encounter a memorable one.
Usually a simple conversion task as well. With such a diverse setting and huge timeline to work with, you can almost surely work in whatever type of technology you want. Liked those plasma pistols from Babylon 5? Well, its very likely that something close to or identical to it was in use somewhere in the Star Wars Universe. Be imaginative.
Starships and Vehicles
Often times this is the most simple thing to bring into the Star Wars Universe. Most Star Wars fans are quite familiar with starships and the technology behind them. Having a good working knowledge of starships in the Star Wars Universe is quite useful when incorporating in ships from Star Trek, for example.
Say if you want to bring in the USS Enterprise into you game (and God only knows why you'd want to), you must first examine some principle differences in Star Trek and Star Wars technology to make correct statistics. The most obvious thing with Star Trek is that warp speed is far less efficient than hyperspace travel. So the hyperdrive multiplier in the stats might be as low as x25 rather than the standard x2 or x1 for most Star Wars vessels. The next thing to notice would be the superior shield technology in Star Trek, so add a few extra points on the stats for the shields. Account for sublight speed and maneuverability as best as you can from your own observations. The Enterprise could probably outrun an X-wing in sublight speed, but lose to it in a long distance run which required a hyperspace jump by the X-wing and a warp jump by the Enterprise.
I won't go fully into designing the Enterprise for you, but you get the general idea. Just use your best observations when designing statistics for the vessel you are crossing over.
I know this wasn't a very in depth supplement, just consider it to be a general guideline to follow to make your job easier. Now get out there, watch some Space: Above and Beyond and make some crossover stats!