Antennas are sized (loaded) by the wave length they intended to be applied to. A CB radio antenna is optimized for 27 MHZ. AM/FM frequencies are are from about .6 MHZ to 1.6 MHZ. FM is about 88 MHZ to 108 MHZ. As you can see the a CB antenna is not the best choice, but they will work for a AM/FM radio (almost anything will). The three practical issues you will run into are:
1. The antenna connectors for a CB radio are usually much different mechanically than an AM/FM radio.
2. CB radios use a coaxial cable for the antenna connection, AM/FM radios use a lesser quality cable.
3. If you use one antenna for both, when you transmit from the CB, you will "jam" RF energy into the AM/FM radio at about a 5 watt level. This could cause damage to the AM/FM radio.
Items 1 & 2 you can "kluge" something up, but it would be ugly. I don't know if they still make them, but back in the 70's they used to make combined AM/FM/CB radios. You can still find them on eBay.
