Forza 5 players can as of today add cars purchased through DLC to their garage. This is available at "no additional in-game cost," according to Turn 10 Studios creative director Dan Greenawalt.
Furthermore, when players purchase a Car Pass, DLC car pack, or a single car, a one-time free addition of each car will be added to your Forza 5 garage. This will be done "without needing to purchase them with in-game credits or tokens."
"This goes for all current Forza Motorsport 5 DLC including cars in previous DLC car packs and it will be the case for all future Forza 5 DLC car packs," said Greenawalt. "In addition, as requested by our fans, this most recent content update makes all of the initial 200 cars that launched with Forza 5 available as rental cars.
"Today’s fundamental changes to Forza Motorsport 5 have been made possible in large part because of the unwavering passion and dedication of you, the Forza Motorsport community. We are a fan-driven series, and the steadfast dedication you have all shown the Forza Motorsport series over the years continues to be a source of inspiration to me, and to everyone here at Turn 10."
Greenawalt went on to state that in early April another announcement about a free add-on to Forza 5 will be announced.
You can read the entire open letter through the link.