ProfileClaimsTransformation.cs

public class ProfileClaimsTransformationService: IClaimsTransformation
{
    private readonly IUserClaimsService _service;
    private const string Issuer = "Claims.Transformation.Service.Authority";

    public ProfileClaimsTransformationService(IUserClaimsService service) => _service = service;

    public async Task<ClaimsPrincipal> TransformAsync(ClaimsPrincipal principal)
    {
        var identity = principal.Identities.FirstOrDefault();
        if (identity == null)
        {
            return null;
        }

        var identifier = identity.FindFirst(ClaimTypes.NameIdentifier);
        if (identifier == null)
        {
            return principal;
        }

        var userClaims = (await _service.FindUserClaimsByUserNameAsync(identifier.Value)).ToList();
        if (!userClaims.Any())
        {
            return principal;
        }

        var claims = userClaims.Select(c => new Claim(c.Type, c.Value, c.ValueType, GetIssuer(c))).ToList();
        claims.AddRange(identity.Claims);
        var claimsIdentity = new ClaimsIdentity(claims, identity.AuthenticationType);
        var claimsPrincipal = new ClaimsPrincipal(claimsIdentity);
        return claimsPrincipal;

        string GetIssuer(UserClaims claim)
        {
            return string.IsNullOrWhiteSpace(claim.Issuer) ? Issuer : claim.Issuer;
        }
    }
}