Well, it's pretty simple. Clients go in the clients/users table. Staff users belong to staff/users table. Invoices go in invoices/invoice_lines/invoice_line_taxes. Everything else follows the same pattern.
You certainly can, and I would recommend it. Though admittedly, we haven't made any documentation available on how the plugin works as we never really anticipated others would want to extend it.
We don't release this kind of information due to typical commercial/IP reasons.
That's for version 2.