There is not currently any way to change the storage location of the Patch Manager packages folder; however, you can mount that volume onto the ~\Server\packages\publisher folder which would cause the package content to be physically stored on another folder.
But there are also a couple of other practicalities to keep in mind. It is not necessary to permanently store the package binaries in the ~\publisher folder. It's only necessary that they be in that folder to faciliate publishing the update package to the WSUS server. Once the package is published, the content can be deleted from the Patch Manager server using the "Delete Content" option from the context menu or Actions Pane. You can maintain a permanent repository of the binaries in another location.
Another practicality is that the amount of content that needs to be stored on the Patch Manager server is not that significant, even if you don't prune it on a regular basis.