A JFFS partition is a partition that uses flash space to make it into a read/write filesystem. It doesn't make sense to "enable it on USB".
If what you really mean is move the content of the jffs partition to a directory on the USB disk, it could be doable, but not practical. The JFFS partition gets mounted far earlier than the USB disk, and is guaranteed to remain mounted throughout the router's operation. That's not the case with a USB disk.