Ah, I meant as making a custom ability with that template, with the field added. I still don't know what sheet you're using, so I don't know what template you're using and how it works, so I have a hard time giving more specific advice. Depending on the sheet, it may already be very easy with some know-how. But, if you post the template into chat, try clicking the chat input field and pressing up arrow - this will either give you the full template the sheet uses, or a call for the template (unless it's a beacon sheet that hates templates and macros). In the former case, you can copy the template, find the field for the modifier, edit it, and save it as a character ability. Ultimately, what you're asking is not a roll20 wide feature, but something for individual sheets, and depending the allowances of a given sheet, it may already be doable.