Recently I needed a quick and low friction way of extending Asp.Net Identity 2.0, tying into the Entity Framework 6 way of working with custom fields and table mappings. To accomplish this first create something similar to a new ContosoIdentityUser which inherits from IdentityUser.
We then likewise create a new ContosoIdentityDbContext which inherits from IdentityDbContext.
Now to stitch it all together in your boot strapper logic, in my case this is within Startup(), just wire up to our newly created extensions:
var userManager = new UserManager(new UserStore(new ContosoIdentityDbContext()));