We are trying to import historical fulfillment information from our legacy system and the csv import process does not appear to handle this. Is there a best practice way to get historical data imported from another system, especially with inventory not in place to handle the fulfillment commitments?

Honestly - I just use a custom non posting transaction any time a client starts talking about importing history anymore.

We were considering custom transactions

if your primary goal is being able to tell auditors what happened or being able to look something up for a customer - it's a decent solution

If you are lot or serialized inventory and want posting transactions, there is no way to fake it and capture the Lot or SN. NetSuite says don’t do it. I would revisit the business requirement and meet it at it’s base level. Similar to what Kevin suggests, if you want sales detail then a non posting transaction is the way to go. If you need total sales $ and no inventory history, then you can use a non - inventory item as a legacy system item and capture the Invoice total on the line. This will give you sales by customer without having to impact inventory.