Ejabberd with existing database

Trying to evaluate Ejabberd as a replacement for our homegrown XMPP server.

We have millions of members and information about their accounts and relationships already exists in a a database(mySQL).

How do you plug ejabberd to an existing schema? for example, I've seen there is mod_roster_odbc to connect to a MySQL, but our data model doesnt map with the one presented by ejabberd (rosterusers).

Should I create a custom mod_roster from scratch?