Hi David,
You want to add one field right. if this is the case why your creating context node. instead of that create one value attribute under any context node. any how that context node is visible in config level and that field too. you can use that field in your config.
go through the below doc in that step 1 will explain how to add value attribute under context node.
http://scn.sap.com/docs/DOC-54345
Thanks & Regards,
Srinivas.