Yes. You can convert a purchase voucher as a whole to a sale voucher.
- Go to Purchase Management > Reports > Purchase register (P > R > P).
- Select the transaction which you want to convert to sale.
- Click on ‘S Sale’ or press Ctrl + S.
- 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: