Hi JKM@SA,
Currently there is no screen to add items to the the ActivityInvoiceType drop down but you can create a screen based on the ActivityInvoiceType table and and use that custom screen to add items to the drop down.
-Michael
Michael Melo
Product Manager
Method Integration Inc.
Website: http://www.method.me
LinkedIn: http://www.linkedin.com/in/MichaelMelo