Create a contact and add an "Instant Messaging" field. Set the protocol to a custom protocol by typing in it (for example: "None").
If you get this contact via the API, you get something like this:
GET /m8/feeds/contacts/default/full/3f9ffe1d8c52ef93?prettyprint=true HTTP/1.1
Host: www.google.com
Gdata-version: 3.0