I do not see much need for such a feature. Why do you not simply put the files on a share in Unraid and then connect the VM to that share? Just for interest what drivers are you interested in that are not available as an iso image?
If you want such a feature then it should probably be done to the KVM team (as that is the software that Unraid uses to run VMs), not the Unraid team as that sounds like the level it would exist at. It may already exist at that level - I am not sure without perusing the KVM documentation.