Yes, you can.

Shopify has recently made changes to the Shopify Admin UI where merchants can no longer set inventory quantity to a negative value.  This is an issue for many merchants as they rely on the negative quantity to keep track of pre-orders (click here to see the discussion on the Shopify forums).

In the API, it's still possible to set the available quantity directly and use a negative value, so our EZ Inventory app will work for this. 

Our app actually only deals with the "Available" quantity and won't modify the "Committed" or "On Hand" values.  All updates are for setting the "Available" quantity only.