You can easily transfer documents between the Costs and Sales Workspaces via the costs/sales inboxes or an item's details page.

To do this, in either the costs or sales inboxes, select the items you'd like to convert using the checkboxes down the left-hand side of the page, then click Move to at the top of the page, then Sales or Costs.

You can also convert an item from the Item Details page, by clicking the Move to button at the top of the page, then Costs/Sales.