How to use custom roles with Azure Active Directory Login

I am currently assisting re-front-ending an existing application that has a legacy "roll your own" role based security system. The customer wants to switch from Active Directory to Azure Active directory as the first step in securing the application (I have this part done). For the role membership portion the customer wants to use the legacy user/role schema (completely custom not a Microsoft role membership solution) for role membership. I have seen plenty of out of the box implementations using Graph and ASP.Net membership w/ Entity Framework but none using a completely custom solution.

Here is the login scenario:

1. User hits website, if not logged in, user is routed to Azure AD for login - I have this done using a Web API project


2. Upon successful Azure AD login, a custom service is invoked passing the email and the legacy user/role schema populates the user object - done


3. Result from service populates the current principal (claims probably) - Not Done