I'm not sure precisely how to implement the upgrade for this... Perhaps it could be an upgrade installed alongside the Tank Upgrade, and allows a tank on the pack to be filled with air, in the same way as filling it with fluid except with the slot being empty instead of with a fluid bucket. Or maybe it would be a separate upgrade item that works completely independently of the Tank Upgrade. But it would make sense to me to use the tanks installed on a backpack as an oxygen supply.

Basically, it would provide the equivalent of Water Breathing until the tank(s) is/are empty.