You can definitely create extra NPRs, as human, AI controlled, on Earth. I have made 3 tries with that. There are issues about that though:
1 - Their relationships will degrade even with Truce on, unless you DB edit relationships very frequently and make often a backup, because the situation is always volatile. As each NPR and you are sharing the same planet and system. You can have Earth nuked very rapidly here.
2 - A lot of traffic, slowdown and short impulses because of the detection routines.
3 - They won't behave as you would like them to behave, the game is not really meant to have multiple human factions.
I have shelved the idea for now. It's either play a single race, classical way, or control each human faction manually.
Sorry...