All the adapters I've seen have been 20v to 18v. Which if you measure the actual voltage, the 18v fully charged is very close to 20v.
I got a 20v 5a battery with Christmas present, leaf blower. I'm not going to buy any more dewalt 18v batteries, which the biggest ones are only 3 amp.
I bought 2 20v 6a aftermarket batteries for less than one 5 amp dewalt battery.
Perhaps you may want to be thinking about converting to 20v. A 20v dewalt charger is only about $30 on amazon or ebay. Then you could use the 20v on a Makita for sure. You'll have to check on porter cable.