David, yes.

1. On the header level map the Document No field to the Sales Order number you wish to update.
2. On the detail level map the fields as you normally would. If you needed to update existing lines you would need to specify the SOPOrderReturnLine field to the value of the line. If you're adding a line, you don't need to set the SOPOrderReturnLine field (IMan will not be able to match the value in the field to a line on the order. When this condition occurs a new line is created vs. updating an existing one).

And yes, please ensure the Update Mode is set to Update or Insert/Update.

Hope this helps.