Dealing with nodes that do not have the field when sorting views using date field
You would need to sort your posts by two criteria


Has a expired date
The value of the expired date


Or alternatively you could use ASC/DESC to display those without an expire date first. Doing that would effect the sorting of the rest of the nodes.

If you want to use two criteria, you could create a computed CCK field that has a value of 0/1 indicating if it has an expire date. You can use that to display all nodes without expire date first, and then sort those with an expired date by the expired date.