Never been able to find it and i've seen it where there's been sequences of different maps (One time without loading) on at least one foundry mission. So to have it optional there could be a trigger put into the map for it. But that's all i've got.
The 'overt' answer is no, you cannot create optional maps
The more advanced answer is, sorta
By that I mean you can have a non objective item a player can interact with that will change aspect of your current map. This would be done via hidden>visible and visible>hidden state triggers. You would be able to (within the scope of the object limit) completely change the layout of a map based on this interaction.
The only thing is though, you would still need to be able to progress your main story no matter which map you are showing, so that is the limitation.
It is not possible to have an actual map that is entirely optional, but...
... A trick some of the more experienced authors will ocassionally use it having multiple maps in one map. The way this works is you have all the assets for both maps on a single map. You start out with all the assets of the first map visible and the assets of the second map invivisible. When you get to the point that you want to transition, you seap it and make all the second map assets visible while making the first invisible. This can be taken further and two different maps on one map and control which becomes visible based on dialog/interaction choices.
If this sounds horribly complicated, it is. All the maps are basicly built on the same space and share the same resource limits. It also only works (for the most part) with custom maps.