What type of adapter should I use to turn a cordless drill in to a corded drill?
No - aH (More commonly Ah since M. Ampere is a proper name, and hours are just hours) is Amperes*hours and is a quantity of charge. 1.2 Amperes for 1 hour, 2.4 amperes for 30 minutes, 0.6 Amperes for 2 hours - all = 1.2 Ah.

NiCads (when happy) are capable of providing very large currents - so you'll need a 12V adapter with a bit more current capability to be able to run your drill at full speed as a "corded" drill. Cut and try is the best I can suggest - i.e., find a 2.5A or 3A 12V DC supply and see if it will run the drill at full load or not - or else find a large 12VDC battery (such as a car battery) and a DC ammeter and see what the drill actually uses for current when under full load. A car battery charger is often the cheapest high current 12VDC supply available, unless you have an old computer power supply with a high current 12V section. The only trick there is that a 12V (nominal) lead acid battery (as from a car) actually operates at a slightly higher voltage than a 12V NiCad battery pack...