Hi! The answer is: probably not. Part of the challenge of the game jam is that everyone has the same span of time to work in. If we allowed people to make assets beforehand, it would introduce disparity between people who knew of the jam 2 months ago and people who only heard of the jam today.
If you were to create a free, generic asset pack and release that before the jam, then that’s a different story. Those assets could then be used by anyone when they started their jam game, which keeps it fair. However, pre-making assets specific for your game just feels unfair.
I know that asset production is time consuming, but that’s part of balancing the 10 days you have to create something.