Question on adding a Map Zone

I have an existing vassal module that includes 4 maps. Only the upper left map (map A) has various boxes and tracks in its upper left portion.

i want to change map A from a map to a map zone so i can create some zones for the tracks and turn off snap to grid in those specific zones.

can i change just map A to a zone without impacting the other 3 maps? if so, how do you suggest I do this?

thanks for any assistance.