Bluetooth would be a good short range option, but like noted, 30 feet or less, not very useful in the real world. There are plug in adapters for most radios, hand held and mobile. Truth is, inside a vehicle it's going to be blocked by the metal.
A "mobile extender" is what you want. A lot of public safety agencies use them. Pyramid is the big manufacturer in this area. Motorola makes some also.
Pyramid Communications Home - Pyramid Communications Vehicular Repeater would be a good place to start.
There are two ways of doing this, in band and out of band. In band is when you use the same band for the hand held as you do the mobile. Out of band is when you us a different band for the mobile and the hand held. In band can be problematic as you need some separation between the frequencies to make it work right. Even then, without expensive filtering, desensing of the receiver can be an issue. Out of band usually works better and will be cheaper, maybe.
Licensing is the big issue. You will need to be licensed for all frequencies you use. There are not any legal ways to get around this. You will also want a hand held that works in a different frequency band than the mobile, if you want to do the out of band system.
It isn't cheap, either. The mobile repeaters can be a few hundred bucks. A few more hundred if you need to buy a new hand held in a different band. Installation, set up and programming isn't something for the hobbyist. If you want it to work right, you need to work with someone who has done these systems before.