Add A Custom Option to a Pick-List Field?

Add A Custom Option to a Pick-List Field?

Is there a way to add a custom write-in option to a Pick-List field?  

For example, the majority of our sales are a set amount, hence the pick list. Periodically we will have a special scenario that would require a custom amount.  I want to keep all the input as numbers for the SUM features in reports.

Can I do this?:

Field : Amount Billed

-$99.99
-$199.99
-$299.99
-Custom (Add this, so the sales person can manually enter a number)

Thanks!