@netravnen afaik pfsense doesn't have a GUI to configure ISIS - but i see that the isisd executable is present on the filesystem. you could try to craft a config manually to get it running.
if you wish to get it supported/included (quickly) in the GUI then you could create a pull request yourself or find a volunteer to create said pull request.
PR's should probably go against the devel branch:
https://github.com/pfsense/FreeBSD-ports/tree/devel/net/pfSense-pkg-frr