You can expose all the functions of the UserStore via your webapi and that would get you a long way toward what you are describing. For the functions it does not provide, you are responsible for instantiating the IdentityDbContext to the UserStore so you can using for whatever you like including listing all users, organizing by roles and so on that an admin interface would require.
I actually implemented a UserStore that called a webapi wrapped UserStore via http to preserve a clients layered architecture. Not difficult at all to do.
Here is a link that discusses more about using IdentityDbContext directly. Why is Asp.Net Identity IdentityDbContext a Black-Box?