Officially you can NOT install pfSense packages on embedded versions of pfSense.
UN-Officially you can and here are the steps:
Go through an embedded install, make sure the embedded install is working properly.
1) Remount the Filesystem RW
mount -o rw -u /
1) Edit /etc/platform, change embedded to pfSense
2) Install needed packages
3) Edit /etc/platform, change pfSense to embedded
5) hope everything still works.