There won't be anything to stop you from purchasing it with an expired license, but note that the add-on licenses have their support tied to the forum license.
In other words, when you purchase XFMG, you will only be able to download the version that was current when your XF license expired. XFMG adds $15 onto your renewal so instead of $40 it will be $55 to renew. So you just have to weigh up whether it makes more sense to pay for renewal and add-on now, or add-on now and renewal later.