JBeam: how to add a new slot to existing part?

Ibishu Hopper with stock frame and body has an option of changing soft top color, which is like a part in the parts tree child to body (like interior color on most cars). However, this option is not available for cut body that comes with crawler frame. In a jbeam file for cut body I've found the following line:

Code:

//["skin_softtop","hopper_skin_softtop","Soft Top/Roll Bar Padding Color", {"coreSlot":true}],

Copying this file with "//" removed into a new zip file as a mod did help, but I don't really like "replacing" the whole body file with a mod, for example in case I will have some other mods for cut body. Is there a way to add a slot to existing body, so that my file would be something like

Code:

{
"hopper_body_crawler": {
    "slots": [
        ["skin_softtop","hopper_skin_softtop","Soft Top/Roll Bar Padding Color", {"coreSlot":true}],
    ],
},
}

instead of replacing the whole file? (exactly this didn't work)