You do want to add it if your going to use it to make bows or flowers. It will help dry it but gives it the stability to hold shape. Don't use it in the MMF to cover the cake.
My suggestion is to add it after your MMF is done and cooled and you are preparing to use it. Not much is needed I don't have that info near me but you would want to check that first. (Sorry). I would take a "block" of MMF make a indent with your thumb in the middle, add gum-tex in the indent and slowly cover it up with the MMF. It is really easy to do this and not have Gum-tex flying around if after your indent is filled with gum-tex, put the whole thing in a ziplock bag and knead inside the bag. Make sure you incorporation the gumtex well. Make sure you keep the MMF airtight when not using.
Hope this helps