Can I skip the downloading of the Windows support software if I already have it?
If you are creating a Windows install disk then you can't unselect the option to download the Windows support software. Boot Camp Assistant, whilst creating the boot disk, will copy the relevant support software into the install media. This process requires a copy of the Windows support software which it will download from the internet.

There is currently no way to use your own copy of the support software if you create a boot disk. If you already have the boot disk, you can just select the option to install Windows, in which case you can use your own copy of the Windows support software.