How to map Sales Order number to Purchase order when convert SO to PO?

How to map Sales Order number to Purchase order when convert SO to PO?

Currently I am testing Zoho Books for my company. 

When create an invoice from a SalesOrder, the number of saleorder SO-0000x will  map to the invoice. 

But when can convert a  SalesOrder to a Purchase order, I do not have the SO-0000x map to the PO. 

Can we also do this?