Yes, your existing specification can be modified to print Cash Invoices.

Modify your cheque specification to add an Advice body (cash) detail section to each Advice. Make the section height for the new sections the same as the Advice body (detail) sections.

For an example of a specification that already works this way, look at the Payable Sample data. AP$SAM01.SAM, Adagio Sample Check 01, is designed with both detail sections.