I'm no radio expert, but I've learned a few basics from a HAM operator friend. One of those basics is that the antenna size and design is a critical part of any radio and they are designed for the frequency range for which they intend to pick up.
On Star uses frequencies in the 824-835 MHz range, which is in the UHF (ultra high frequency) spectrum. The radio wave is very short (hence being called "short wave") so the antenna is designed and tuned accordingly.
CBs operate around the 27 MHz band. This is a comparatively long radio wave so it requires a very different antenna size than an On Star antenna to be effective.
So, even if you found an adapter to connect the antenna cable, it wouldn't work very well at all.