If you are transferring your accounting to Simplbooks and you have some claims or liabilities from previous periods, they should first be reflected in the initial balances and then the relevant documents should be entered under sales invoices or purchase invoices as usual.
1) To enter initial balances see the manual Entering initial balances.
2) To enter purchase invoices and sales invoices see the manual Purchase invoices (creating purchase invoices) and Sales invoices (creating sales invoices).
N.B. Purchase invoices and sales invoices should be entered under dates earlier than the entry dates for initial balances.
That way no financial transactions are created for those documents, but payments/receivables can be bound.