Set a category default to never expire: this applies to only newly entered items.
Navigate to category settings then add or edit a category to display the screen below:
With this setting checked when you add inventory under this category the expire date for consignment inventory will be blank:
*You can not move inventory from a category that has this setting turned on to a different category that has it off, then move it back to the original and expect the item to go back to no expire date - only newly created items that stay in the category will never expire.