After a long wait, PDP Gaming launched a limited quantity of Rock Band Wired Legacy Adapters for Xbox One. Fans have been waiting for this for quite some time.
Per the product webpage:
“Bring your legacy controllers to the show with the Rock Band™ 4 Wired Legacy Adapter for Xbox One. Officially licensed by Microsoft, the USB Adapter is exclusively compatible with Rock Band™ 4 game software and these specific legacy controllers: Rock Band 3 Midi PRO-Adapter for Xbox 360, and the ION Drum Rocker for Xbox 360. Simply connect Legacy Controllers to the Xbox One with the slim and hassle-free Rock Band™ Wired Legacy Adapter for Xbox One and you’ll be rocking out in no time.”