No problem at all.
You can do this without custom firmware by setting Content Share = On in the HDR-FOX and then accessing it via Media >> Storage (blue) >> Network on the HD-FOX, but the functionality is limited to playback. With custom firmware, set up auto-decrypt on the HDR-FOX and load the foxlink package both ends - the HDR-FOX will appear as if it were a USB drive to the HD-FOX, with full play/resume/bookmark/move/delete capability.
Note that HD-FOX is no longer in production. I think some did turn up on Humaxdirect a little while ago, otherwise it's eBay.
PS: I have three HD-FOXes accessing two HDR-FOXes, but I use network-shares-automount because of the tweaking. foxlink uses NFS, which does not make any USB drives attached to the HDR-FOX visible to the HD-FOX, but is simple to set up. network-shares-automount can use SMB or NFS, and SMB by default mounts everything visible in the WebIF media browser under /Media - ie USB drives as well. It isn't all that difficult to set up, but some people have reported problems. The HDR-FOXes have access to each other as well (but one has to be careful to avoid an infinite loop).