Unfortunately you are not allowed to modify existing fortnite assets,which includes the meshes,unless the aforementioned prop already has multiple components (rare) like eg this covered up zero point prop which has multiple components, but again its very rare for a prop to just not be one mesh.
Most people use porting tools to re-import the props as their own to then edit, Although the drawback would be that they will then start taking away from the download size of your project since users will need to “download” the imported assets