I found a way to give players an additional maximum level. If you try to give a level 6 player a stat upgrade the game cannot validate it, however the player still gets spp for it. If you try and level them up enough, then go and skill up another player and come back to the first player, you can skill them up twice, above their supposed maximum level. I have attached a video of me doing this and creating an entire team of level 8 players.
Possible to create level 8 players in custom team mode.