Well, I suggest not trying to 'convert' them (I love PF, but that is a different subject).
Trying to expand their gaming experience is a much better tactic.
A second game sounds great. Maybe a one-shot session with some interesting pregens, that highlight some of the things that HARP has, that PF doesn't (The racial mixing is different, as well as having the casters with spells that PF casters might not have, for example).
Years back, a friend and I went to a local Con, hoping to play RM. We found a guy who ran MERP, as well as D&D. He let us vote, as a group, 4 people wanted D&D,2nd Ed, we wanted MERP.
So... he had us all make 3rd lvl characters in our preferred system.
Easy enough on the fly, with Orcs,Goblins, etc...
While they attacked and hit, doing D8+3 or whatever, we did crits with ongoing effects. The D&D players were fascinated by Stun, bleeding and things like tendon damage. Likewise, the skills, Open-Ended rolls...Not to mention Healing Herbs.
It did go bad for us in one area. We took a fall...While they all took 2D6 damage, we took a +20 fall/Crush. Ouch. My friend's character ended up with a broken leg, and I with some minor wounds,whil they all dusted themselves up after the fall... Still, it converted some folks, and encouraged them to try the new game.