Boot from SD card is not supported. Also, for a given image, the partitions on the internal flash are set at flash time, and are very tricky to change afterwards.
My recommendation would be to create a large linux partition(s) on an SD card, copy everything in /usr to that portion, then change your fstab to mount that external partition as /usr at boot. This means that the SD card has to always stay inserted, though. Would that be such a big issue?