nope,
probably not the way you seem to be wishing for.
"Subgames" are collections of mods similar to a "Modpack", so yes you can combine the mods of two different subgames into one subgame. Of course you will have to watch out for name-space conflicts. for example you could have several "mobs:sheep" that try to use different models or textures or have conflicting apis. I presume though this is not what you have in mind as you do not mention a combined gameplay but rather still seperate worlds with different features.
You seem to think of the old and often asked for feature of being able to switch to a different world using some ingame feature. So to name it, you want to build a portal to be whisked to some other realm that is not part of the map you have built the portal in. Yes, Minetest does have
Nether or
caverealm mods just as it has several mods that fill the sky with landscape to explore such as
Skylands or
Moon Realm. All these places are still physically part of the same map database though and could be connected by building long tunnels or high towers if you don't put indestructable bedrock or cloud like nodes between them. To switch from one world database to another, be it locally in singleplayer mode or between remote servers in multiplayer mode, at the moment the main menu is the only possible way.
This more recent discussion might highlight some of the problems to solve:
Different "Dimensions" / Other worlds #4428