There are some ways to make this, but first.. it's not possible to add another "build" menu itself..
The first way would be making a spellbook with all the buildings in it, as abilities without any function.. so they do nothing if you click on them.. except using it for our trigger that checks if you got enough money to buy it..
Another way would be replacing your builder with another one.. like you have 2 builders, one for defense structures and one for offense structures for example.. add 1 ability for each builder that contains some information like "Press this to change to offense builder menu" as you clicked the ability the builder gets replaced with the other builder.. and so on and so on.. think you get what I mean..