Exhaust adaptors

On top of all the other bits i'm grovelling forhelp with.....

is there or does anyone know of an adapter that allows the original stock bolt fit exhaust to an aftermarket U-Bolt type fit..?

something like a bolt flange with 3 inches of slighly smaller diameter pipe that the replacement BACK BOX can be slid over and then U-Bolted to it so i can remove the oem for a replacement without having to replace the ENTIRE system

Thanks in advance

Stu!