Yes. You can convert a purchase voucher as a whole to a sale voucher.

1. Go to Purchase Management > Reports > Purchase register (P > R > P).
2. Select the transaction which you want to convert to sale.
3. Click on ‘S Sale’ or press Ctrl + S.
4. It will generate a Sale Voucher with all the data of that particular Purchase Voucher.

Note: You can do similar thing to convert Sale Voucher to Purchase Voucher, using a similar proces. (use '^U Purchase' button in Sale Register). 

For more info on Purchase Management please view:

• http://www.rancelab.com/help/fr6_purchase_management.htm