You can actually add Ars Magica now, Mithion changed the permissions required on the Ars Magica Minecraft Forums post, you can now use it in packs, as longs as you follow the conditions set here on the post
Thanks for letting me know! I'll look into it by default for 1.6- maybe I'll make it disabled (just like I'm going to do with MFR) by default next update.