Create custom Grand Prix using addon tracks [SOLVED]

Hi,
I tried to create a custom Grand Prix with tracks I have downloaded by the addon manager. The .grandprix file works perfectly if I insert tracks that where distributed with STK. However, using a downloaded track results in an error and the Grand Prix does not get listed ingame.
Is there a possibility to use addon tracks in a custom Grand Prix?
Cheers
Johannes
I tried to create a custom Grand Prix with tracks I have downloaded by the addon manager. The .grandprix file works perfectly if I insert tracks that where distributed with STK. However, using a downloaded track results in an error and the Grand Prix does not get listed ingame.
Is there a possibility to use addon tracks in a custom Grand Prix?
Cheers
Johannes