Will these Arx be purchasable through Steam?
I'm sure FDEV will be along to clarify but...
Steam users currently have their very own mirror of Frontier's Store from which you can purchase with your Steam Wallet funds. I would imagine this (like Frontier Store) will simply switch to offering ARX bundles which you can purchase with your Steam Wallet funds.
EDIT: Actually it seems Steam already supports the functionality for purchasing varying quantities of premium currencies as a DLC. A recent example would be RAGE 2's - RAGE Coins. I expect this will be the method employed and the store variant retired.