Ok, so yes, as JGF described, there is a line in the Veh file of each car that specifies which class the car is in. Navigate to Gamedata/Vehicles and then into the mod you want to add. Look for the veh file of the car you want and open it in notepad.
There are two ways to do this. You could take note of the classes that are in the mod you are running and add one that is common to all into the car you wish to add. With a bit of patience you can add any cars you like into a race. But they will all be racing in the same class.
The second way is to add a class into the rfm file. This method allows you to create multi-class races and most importantly, add individual car mods into an existing mod. It is recommended that you explore this method to better understand how it works.
Navigate to the RFM folder and look for the rfm file of the mod you are running. Open it in notepad.
Look for a section that looks like this image.
In the Vehicle Filter section, we specify the vehicle class that was in the Veh file.
To add an additional class, copy the section, including the { Parenthesis and scoring info.
and paste it in to create a new section, change the class in the Vehicle Filter.
If you don't want separate classes, simply add the vehicle class to the line Vehicle Filter.
for example:
My instructions may be confusing, but don't worry, feel free to ask more questions as we will continue to help you along the way.