You could use Task.Run() to start and get a reference to a new Task:
public IdentityResult Validate( AppUser item )
{
     IdentityResult result; 
     // do stuff
     return result;
}
public override Task<IdentityResult> ValidateAsync( AppUser item )
{
    return Task.Run( () => Validate(item) );
}
solved how to return method which has task