You're allowed to use extensions, whether you made them yourself or not. 👍
We have a rule on the jam page that explains what amount of "pre-existing content" is allowed to be used in your game, since remaking the same extensions/assets again just so you can use them in the jam would be boring and you wouldn't really learn anything, but copying over too much from existing work you've done would be unfair for other participants.
I also pinned a message in the GDevelop discord game jam channel that hopefully explains pre-existing with an actual jam game example:
https://discord.com/channels/258623956158906368/630511209778774037/1222146236325...
Hope that helps. 👍