That's what the supply challenge is, which you find under Play->Custom Scenarios->base/supply.
Freeplay is supposed to be, well, free play. You're not bound by any goal, except for the rocket silo (and even then not really). Putting this additional challenge at the end seems to me kind of forced ...

I would like to propose an extended game-play for somewhat of a post-game challenge.

The game could come up with progressively more difficult production demands your expected to meet, and then maintain.

Example... you launch the rocket then the game could pop up and say...

"Produce 100 generators ...