Perhaps allow people to buy Iron Coins for Frontier Coins in the store?
eg: 250 IC for 50 FC, Limit 1 per week/month.
FWIW: The above cost is sort of based on the cost of Skill Shards, but rounded down to 50 FC's worth. 350 IC for 1 Skill Shard vs 350 FC for 5 Skill Shards (basically works out at 70 FC per Skill Shard).
I've got all the characters out of the Frontier tab of the store, Got a stack of Skill Shards ready too, and basically I'm just hoarding most in case something else comes along in the future.