How to assign roles to already created Users in ASP.NET Identity?

In ASP.NET I am trying to attribute Roles to specific users in my Database, I already have the users and the roles created in the AspNetUsers and AspNetRoles, but now I would like to attribute each to the other as I wish. Is it "safe" to do so manually, through the SQL Server Object Explorer? Will that work with the Authorize DataAnnotations later? If not what should I do?