How do I add options to a drop box?

I need to add additional options to a drop box located under Contract and Rate Management -> Service Provider Manager -> Edit Service Provider -> Communication and Remarks Tab -> Service Provider Insurance Policy section -> Policy Type.  The box currently only has options for "Cargo" and "Liability".  I need several other options to appear in this drop box so we can properly store data pertaining to multiple policy type effective and expiration dates.  I cannot find a way to edit this field via Manager Layout for the SERVPROV screen.  I am unsure where else to look.  Any suggestions to