So I plan on beginning in sandbox mode so I can edit player ratings/coach names etc. then convert into career mode but I’m just wondering if it’s possible for me to be able to export/save the file that i edit in sandbox mode so I can use it in the future if I want to begin a new game rather than have to do all the editing again?

I would save the sandbox league as a different name before converting to career so that you can go back and use it again. Exporting/importing only works for players.