It might be because those cars are not eligible to be turned into Works cars. You can check if your car is compatible in the "Buy Lot". Go to your car, and check the upper bar. There may be a total of three icons: a gas tank, a "W" symbol, and a drift symbol. The car must have the "W" for you to be able to use it in Works races.
"Works conversion" is in the upgrade menu. If you've done all the upgrade packages required the "works" should be highlighted (selectable) on your bottom right screen. On PS3 you had to press Select..