Any new unit can be incorporated into a scenario with the following methods. In each case you bid for the right to gain the new unit. The bids are by individual nation. You can bid more than your available money (but you will not be able to buy anything into you are back in posititve numbers). Roll a D6 to determine what turn it becomes available.
Option 1 The successful bidder receives the unit.
Option 2 The successful bidder receives the right to build the unit.
Option 3 The successful bidder received 1 unit each turn.
The bidding system could be applied to technologies or even rules changes (such as the ability to retreat as the defender).