You can't.
What I always do personally is this. Lets say I have a marine, 10 base damage with bonus of 1. Start them at 3 upgrades but with base damage 7. Upgrades for 4-255 don't require the science facility.
The downside is that the cost factor is already put in three times and your units need to deal atleast 3x the bonus to start off with. Fits in great in a lot of cases, though.
EDIT: Wow, 3 posts in the same minute time frame to a random question in mma. No worries, kow