Chuck,
you have a few options all are from custom barrel makers.
1) is a barrel from OTT, they use an off set barrel so you will not have to do anything to the frame. they will make you any rimfire barrel you want.
2) is from bullberry barrel works. with this you will need to replace the firring pin / breech plug with a custom one. this is nothing hard and only takes about 3 minutes ( you should remove the breech plug every so often for cleaning anyway) then order your rimfire barrel from bullberry.
3) I think at least one time Midway usa had the rimfire adaptors listed and I am sure SSK will be happy to make you a rimfire barrel
You can replace the breech plug at will with no modification to the frame. I doubt it will void the warranty