The names for randomly generated planet are stored in the English.str file. So technically, modding them would be a possibility.
However, two questions arise.
1) How many entries can the engine handle?
I have a compilation of star names (both real and fictional) from GalCiv2, about 2600 (!) of them. It'd only take an hour to code something that puts that list into the required format, but I'd rather know if it'll work before I start on that.
Same goes for capital ship names. There are great compilations for SpaceEmpire Design names out there.
2) Can this be a mod? The english.str doesn't seem to be part of the 'clean' slate that came with the mod tools. Is that only because they're still not really released, or because that file can't be modded?
I'd ask for planet and ship names be externalised into separate files so we can easily customise them, but it's probably a bit late for that.