As far as I'm aware the only actively developed 5e sheet is the By Roll20 one, and individual users cant modify that in any way because the code isnt accessible to use. If you're okay with an older version of the code, you could grab one of the sheet versions ported before the code went private (Darker Dungeons is one such), and try your hand at modofying that. If you give it a try, post specific questions about what you're trying and we'll help.
