What happens if you bomb your target with objects made 99.99999 to 100% of hydrogen? In so far as I know, much of what makes up a the asteroids you are using to bomb your target into "star-hood" is silicate and iron. Whether this is true in the simulator or not, in reality, iron is death to a star, because trying to fuse it into anything else will result in a net loss of energy, core collapse, and a nova or supernova. While only stars larger than about mid to late type B are massive enough to ever reach the point of creating iron "ash" in their cores, and, therefore, going nova, I can only guess that injecting massive amounts of iron into your star--even a small red dwarf, would have a similar effect.
As to "silicates", that's just a bunch of rock which would (depending on how Ubox is programmed) end up basically just contaminating your star and making it putrid.
I know that a few times I managed to bombard (or just dynamically increase the mass) of Jovian planets until they became small suns. Jovians are made mostly, if not virtually entirely of hydrogen, so it seems to work rather well. I've never tried to turn a rocky world into a star, figuring all I would get was a very hot rocky world.
(I did manage to lose a rocky world and finally found the silly thing hanging around in the core of a sun, but that's another, rather embarrassing story....and it only got to about 5000 degrees K, which made no sense since that particular sun (presumably) had a core temperature of around 16 to 17 million degrees K, being of type F8V.)
A./