One way of doing this is subclassing OAuth2ServiceImpl and overriding the sign method to add the missing parameter.
Then from your FeedlyApi
class (that inherits DefaultApi20
, I assume), override the createService method to return your service subclass.
This is a bit of a hassle, and maybe it's easier to do it from scratch. OAuth 2.0 having so many variations and being significantly easier than 1.0a is the reason why scribe is going to be 1.0a only in future versions.