Perform a shutdown -r -y 0 when you can take the system down. When the system is coming up, interrupt the boot process when indicated. You should be brought to the step where you can specify where to boot from. Just enter a bo pri, and when the system asks you to interact with ISL, enter Y. Once at the ISL prompt, you can issue a hpux -is to enter single user mode, in which you can execute your expansion of /usr.
lvextend -A y -L 20480 /dev/vg00/lvol7 && \
umount /dev/vg00/lvol7 && \
extendfs -F vxfs /dev/vg00/rlvol7 && \
mount -F vxfs -o ioerror=mwdisable,delaylog,nodatainlog,dev=40000007 /dev/vg00/lvol7 /usr