I'm not sure if it's possible, but if there's a way to do it, you'll find it in the apt howto
. I'm not sure this is a good idea though. It could quickly become confusing trying to keep track of which packages are installed where.
A better way to solve your problem would be to move your entire /usr directory onto a separate partition. Simply create a new partition, move all of the files in /usr into it (be sure to back it up first), then edit your /etc/fstab so that that partition mounts at /usr. If you do it correctly, the changeover should be transparent to your system.