Can I use downloaded custom post processing shader ? Like a tilit-shift shader that i used in another project. (I never programmed any shaders so it would be impossible for me to make it on my own in this timeline).
No. But Unity has loads of built in shaders that you can use. But if you need a really niche effect, then you need to make it yourself. If it's going to take too long, then it's too complex for a game jam anyway.