Short answer is the obvious way to disable permanently the driver enforcement check. So you can install a modified driver. A typical example are Nvidia or Intel drivers, that usually are artificially blocked on some OSes. Say intel drivers meant for Vista cant be installed as they are in 7/8/10/11, but is deadly simple to edit the infs to make them working in the newer OSes. But given you lose the signature in the process, they won't install unless you disable (manually) the driver enforcement in the boot manager on each boot. OR More conveniently you enable the test mode.
It depends on what you mean with unsigned drivers. You still need a cat file listed in the info, and a singned sys, then the cat can be wrong and the signed .sys can be a selfsinged file. You still can't install driver that lacks one of the above (typical example a stock driver taken from the driverstore folder of another windows.) Use dseo13.exe to easily selfsign an unsigned sys.