Takes too long, probably.
We could technically "grow" oil too by putting biomass far under the ground and wait a few million years, but it's just not economical.
The Empire is already falling apart, they probably don't have the time to terraform a planet to sustain a life form that isn't native to it and set up an entire eco-system with the right balance and nutrition for it to be stable and thriving.
Humans already have a lot of problems with growing and keeping simple bacteria colonies stable for use in chemical processes. I imagine a world spanning system with many co-dependend species would be an absolute nightmare to set up and be made to thrive.
Not to mention they would have to wait a few hundred years on top of that for a large enough population to engulf the planet.
It's much easier in the short term to just find a planet that's already naturally formed and is ready for the taking. In the long term it means they are fucked though, since the resources they need to sustain their growing empire are basically dependant on being lucky and being able to keep finding planets that are ready and defenceless, but many great empires fell because they grew too quickly and too large.