The best way is to do an import into Inventory Transaction Entry set as Transfer Type of transaction.  You will need the FIFO tier, Lot Number and Serial Number in the file.  Don't try to get cute with setting up the warehouse as customer and using a sales order and you are just making it more difficult on yourself.  

I agree completely with BigLouie. Stick with Item Transfer transactions.

To create shipping documents we've designed custom reports to look at unposted transfer data (linked to the warehouse table for to / from addresses).

Thank you.  Appreciate your input.

Kevin, do you use Sage to create your shipping documents, or do you have external templates that you use? 

We make a custom designed Crystal Report, according to whatever the customer needs.

Just thinking out loud.  Always wondered about using a material requisition in PO as a formal way to initiate a "transfer request" and create shipping documentation, close the requisition to ship and remove from the warehouse, then do receipt in inventory transactions in the new warehouse when the product arrives, referencing the requisition #.

IM Receipts are posted against the Purchases Clearing account... and you have to process two transactions (plus a potential GL adjustment to correct the Purchases Clearing account).

I've considered creating an SO Quote (which does not affect "OnSO" quantity), printing shipping documents from there, then have a button script to copy the quote line data into an IM transfer (and deleting the quote)... but setting all that up is more work than just designing a custom report to print from Transfer data.

the Requisition would post against the clearing account to provide the offset debit entry.  But I could see it getting troublesome with the extra entries in the account when balancing.  Your clearing account would be off by the un-received transfers. But you could post to a UDF to make it easy to track and segregate warehouse transactions hitting that account.  Again thinking out loud.