Even if it’s an older form, you should not need to open every field. FormMaker will automatically recognize existing form fields in your InDesign document and carry over the formatting and any default text, etc., that they contain.
Of course, they do need to be proper form fields in InDesign, not just text frames. And obviously some sort of text styling (and/or default text) has to have been applied to them.
If it’s still not working, feel free to send me the InDesign document privately and I’ll take a look.