Hi there,
On the rules page it outlines that free to use, or public domain assets are allowed to be used, but if these are assets from a previous wip project they aren't allowed unfortunately.
A good way to include premade assets is if you upload them to itch as a free bundle (so anyone can use them) and then they can be used in your project. From the page:
"Only new projects are allowed. Current Demos, WIPs, etc. will not be permitted for the jam. You're allowed to write outlines, draw concept art, look for teammates, save/ bookmark resources and website and other actions that don't contribute to finished assets."