Unfortunately no "real" cars can be included due to legal complications and copyrights etc. It will however be possible to create custom cars to be raced in Performa Cars. Since additional cars will not be sold or economically gained from, I believe people have the right to build their own cars to race, based on fantasy figures and images - or real ones. Custom cars can also quite easily be converted from "GTA" by importing and resizing the images according to step 1 below.
The CarEditor.exe executable is included in the game folder. Launch it and look at the included cars before you create your own to see how they are configured.
Since the game is in alpha-stage no proper car tutorial has been created. It'll take some trial and error to make your own cars. Feel free to contact me if you want to share your cars via this website =)
In addition to the general car data, parts can be customised for each car. Parts can be created that will affect Performance (such as turbo upgrades, ECU chips, exhaust systems, intercoolers etc), Grip (such as tires, suspension kits etc.), Weight (such as weight reduction kits etc.), Drag (such as lowering kits, custom bodywork etc) and Damage Level (such as chassis reinforcement etc.) All these items are added through the car editor, but it is always good to have the data handy once the editor is released.