This involves changing the IDs of the teams, which in turn means changing the ID's of the drivers, which in turn means changing the IDs of the suits, helmets, liveries, ....etc
It's a large process and can be very messy if not done exactly right, which is why most people don't do it.
If you download the database from my mod, you'll have all team offers after season 1. It's a "better than nothing" scenario. I simply adjusted the "driver_rank_required_points" table to make you a rank 50 driver right away, so by the end of season 1 all teams will make you offers.