Create an USB stick (or disk) with the Freebsd image you want to boot and connect it to a USB port on your Pi.
Create a small standard Freebsd image on a small SD-card and place it in the Raspberry SD slot.
Boot it and create as superuser the file:
/boot/loader.confwith the following content:
currdev=disk0s2aNow reboot and your Pi will boot the image on the USB stick.
Note that when you create a standard image on the stick, that it will have two partitions. The first FAT partition with the firmware and config.txt is not used in this setup. The firmware and config is always loaded from the SD card. You can remove this unused partition (and replace s2a by s1a in loader.conf) or replace it by a swap partition or just leave it as it is.