There appears to be no mention of this in the rulebook. The original, vanilla Catan game rulebook states this:
If there are not enough of a given resource in the supply to fulfill everyone's production, then no one receives any of that resource during that turn.
So that's an option. Another option might be a house-rule along these lines: A player should receive Wood, but there's no more Wood in the stockpile. If that player gives a Wood from his own cache to the bank, he gets to choose another resource. This is basically a 2:1 trade, giving two Wood (one from their hand and one owed them by the bank) for one other resource. This keeps the player happy for he gets resources, and puts some of this resource back in the bank.