I think this method will work to flash a new radio without affecting the rom. I installed the Mexico Retail radio onto Holoblur without affecting Holoblur at all.
Assuming you have a Windows machine:
1) install the adb/fastboot files to a single directory on my computer;
2) place the NON-HLOS.bin file you got from the desired rom in the adb/fastboot directory from step 1;
3) open a command prompt window as an administrator;
4) install the Motorola USB drivers and connect my booted phone with USB debugging enabled;
5) from the command line, type "adb reboot bootloader" and press the enter key to get my phone into AP Fastboot Flash mode;
6) from the command line, type "fastboot flash modem NON-HLOS.bin" and press enter; and
7) reboot the phone and check to made sure the baseband is the one you wanted.