You can't with only those varieties available. With time and generations you can create Sweetgrass by breeding a Bronze tom with your Royal Palm hens. It would be even easier if you had a Black Wiinged Bronze to start with but would still take generations and the really difficult part would be determining which male offspring is pure Sweetgrass (not carrying the Narragansett gene (n)).I have BR, Nari, and black Toms and I have RP hens. How does one create a sweetgrass variety?
https://www.backyardchickens.com/threads/sweetgrass-turkeys.299873/#post-10918826