Yes, it is possible. Here’s the docs: http://clusterlabs.org/doc/en-US/Pacemaker/1.0/html-single/Pacemaker_Explained/index.html#s-add-heartbeat
You probably don’t want to do it that way unless you carefully read that whole document and know what you are doing. Unfortunately, there are also currently issues with HA backup/restore, so if this is a live system, I suggest waiting until freePBX 13 HA backup/restore works properly.
If you must, fully update the machine, backup the machine, clean re-install on same machine, activate (you will get your licenses back), update to same level, copy backup file to /var/spool/asterisk/backups, apply backup, immediately go to advanced settings and select view and override read-only settings, submit (NOT APPLY), then find the block “Remote CDR Database” and clear out all settings. Now apply config. There will be a few left-over issues in your freepbx_settings table.
Fix them at the command line with:
# fwconsole chown
# amportal a s
# fwconsole ma refreshsignatures (repeat until it doesn’t change anything)
Then create the cluster with separate interconnect NIC.
Then fresh install, update freepbx-b and join to cluster.
Good luck! 