Using cfdisk or some other tool, split hdb1 or hdb2 into required partitions.
Mount the new partition as /usrnew - say
Using `find` all in /usr, piped to `dd`, copy all /usr to /usrnew.
Change the mount point of /usrnew to /usr.
Rename /usr (under hda2) as /usrold - say.
Reboot the system to be sure (or umount and remount).
After a reasonably safe period, remove /usrold.
I expect that the whole thing can be achieved by your favourite GUI tool.