Creating a character sheet as a non-pro member

Dose anyone know if there is a way to code a character sheet and use it in Roll20 without being a pro member (e.g. building the sheet outside of the site and importing it)?

The CREATOR of the game would have to be a pro subscriber in order to allow for a custom sheet to be used in a game. However, there is nothing to stop you from  building a sheet outside of Roll20 and submitting it to the repository on GitHub to be added to the community sheet list which would then be accessible to all users regardless of subscription level. If you choose the latter I would recommend befriending a helpful pro who will assist with in app testing as that is the only true way to know if the sheet is ready or not :)

Thanks, I assume if I got a month of pro membership and created a character sheet (and submitted it to Git/hub) I would still be able to use it if I went back to free membership?

100% correct, as long as it was merged into the main repository, as you rightly state, the whole world would benefit from your dream :)