You can use ImportXML VBA command it import XML forms into Access.
Each group becomes a table and each field a field in that table.
You might instead be able to use VBScript/JScript and use DAO or ADO connection methods.
But I'm pretty sure that you can't connect the primary datasource of an Infopath form to a database after the fact - it has to be created from the beginning as using the database as it's primary datasource.