You can, but you're looking at something complicated. Your SD can be treated as your /bin and /usr/bin partition by using it for their mount point, which would shave off a lot from your USB, but you're going to have to modify /etc/fstab accordingly.
https://wiki.archlinux.org/index.php/fstab
Your entry would look something like this:
Code:
/dev/sdb1 /usr/bin vfat* rw,relatime,data=ordered 0 1
*vfat only if using FAT32
You'll also need to migrate the entirety of these directories to your SD since you didn't do this during the installation of the OS.