In an article posted to his site Rob Allen shows you how to hook in the OAuth2 authentication for an Apigility-based application with a pre-existing database table structure that may not match the defaults Apigility is looking for.

I have a client that's writing an Apigility API that needs to talk to a database that's already in place. This also includes the users table that is to be used with Apigility's OAuth2 authentication. Getting Apigility's OAuth2 integration to talk to a specific table name is quite easy. [...] However, if you want to use different column names, that's a bit trickier as they are hardcoded in the OAuth2StoragePdo class. To get Apigility's OAuth2 components to look at the correct columns, you create your own OAuth2 Adapter. I chose to extend ZFOAuth2AdapterPdoAdapter which extends OAuth2StoragePdo and go from there.

He includes the code for this extension of the PdoAdapter (a "OAuth2Adapter" class) in the post showing the definitions of the get user, set user and check password methods the OAuth2 flow needs to match users to OAuth sessions. He also includes the code for the "OAuth2AdapterFactory" class that's used to pull the custom PDO adapter class into Apigility and, along with some configuration changes, make it available for use. Then it's just a simple matter of changing the authentication type in the Apigility UI.