Yes, Formstack for Salesforce works with Person Accounts. You can enable Person Accounts in your own Org which basically combines the Account and Contact objects into one with slightly different data points. It is literally 2 records merged into one (with 2 record IDs) as a junction object.
To add a Person Account to the form you would use the Account object.
A form with Person Account details can be prefilled using either the contact or account data.
One caveat to be aware of is that if a Person Account is related to other Person Accounts, those related person accounts cannot currently be displayed on our forms.