Most of the 12v fan specs I have seen max out at 15-16Vdc. Now you could probably run it for awhile at 17 but the lifetime of the fan would be short.
There is a -5V power available but it will only supply a few milliamps of current. If you hooked a large fan to it there is the good possibility of burning the supply out.
In short I wouldn't recommend it.
It would be better to run a higher CFM fan at 12v then to overclock your fan to 17v with the dangers to your equipment.
If you want to run at a higher voltage you can find variable power supplies for sale. Depending on the wattage they are sometimes reasonable in cost. Then you could run the fan from 7 to 15v with some safety.