No, you don't. you need the CON resigner to change the file and make it work again. you can change it all you want without a CON resigner, it just won't work
You know what would be a cool map? A canvas of the pit! There are 2 barriers and a crate that you can't normally delete, but i made a modded usermap where they are gone. If you want, i could upload it so you can resign it. It would be the first ever blank forge variant of the Pit!
You know it's possible because other people have created modded maps like 'Valhalla V1 and V2' for example. It's definitely possible... ...just not many people know how...
If you use this method, then when you next want to spawn a mongoose, you will loose the mod. What you need to do is swap the 8-byte hex string after the mongoose tag with the 8-byte hex string after the warthog tag (it's already in there). This way, when you save the map after playing it, the mod will still be there!