Can I upload an existing form

It depends on what file structure the form is created.

If the form is in Excel (BIFF) .xls, Excel 2007 (OfficeOpenXML) .xlsx, CSV, Libre/OpenOffice Calc .ods, Gnumeric, PDF, HTML, then you can use this app to import and convert it to a JotForm form: https://apps.jotform.com/app/spread2jot.

However, please note that it cannot copy the form exactly at it is shown, like some complex fields or its design. It could copy some of basic fields though.

I strongly suggest recreating the forms using the form builder, it's easy as our form builder has a drag and drop functionality. Let us know if that works.