This should be the standard approach to any startup that is doing process automation. The MVP for this should have been just getting the users email/password and then a big text area where they enter their stuff. Tell the users you are doing it by hand at first to learn the best way to do it and play that up as a feature.
Might not applicable for SaSS. Say you're building an accounting SaaS. You can probably type their input manually ("Excel-backed or plaintext-backed"), but I don't think your customer will like you seeing their transactions.