Historically the Germans acquired 244 Pz35(t) when they occupied the Czechs. Although the design of the Pz38(t) had been completed and a number ordered by the Czechs, none had entered service in March 1939. Thus all production of Pz38(t)'s was undertaken by Germany using Czech factories and the player should not get any of them upon annexation unless the Czechs have started production earlier in game or the annexation comes later than historically. The Slovaks acquired 52 Pz35's of them when they declared independence, so the split between Germany and Slovakia for Czech equipment upon annexation could be 82.5% - 17.5%.
While the Pz35(t) could be considered a rough equivalent of the Pz II, the Pz38(t) was better. It's been a long time since I've seen anything from the devs about experience upgrades to tanks, but I think there are 2 ways to go about adding Pz 38's to the game. Firstly they could be the experience upgrade to the Pz II light tank. The only problem with this is that the Pz II chassis remained in production until 1944, and if the Pz38 is just an improved Pz II the small loss of production efficiency in changing would mean that it would stop being produced quite early in favour of the Pz38. The other alternative is to rework the tank tech tree to add another light tank model between the Pz II (designed 1934) and the Leopard (designed 1942), and for the German tree to call that model the Pz38(t).