I am pretty sure that you can extend the game as many years as you wish, by changing one varible in the source code and re-comiling.
If you look at th source code file "uqm-0.3/src/sc2code/globdata.h", near the top there is a varible called "YEARS_TO_KOHRAH_VICTORY" which is set to 4, which should cause the game to finish in 4-5 years, I think. I would assume changing this value to, say "10" would give you an extra six years. I could be wrong, but that's my theory.