Taking a P100/HT50 out of band is VERY simple.
Unlike later Motorola products, the archive file is actually a straight ASCII file. Just program everything except the freqs like you want it, and save the archive. Use your favorite text editor to fix up the frequency info and save it.
Then, read the archive with the RSS and, without making any changes, program the radio.
The only bad news is that the VCO is a sealed unit. So, some P100's will go farther out of band than others, and there is no alignment "tweak" to extend the range. Most will do at least 2MHz and some as much as 10 or so.
Channel Expansion
It is relatively easy to convert a 2 channel to a 6 channel. If, the unit in question uses the 2/6 channel controller.
Be aware, however, that there were a few units built with a 2 channel only controller and cannot be converted.
The 2/6 channel controller is a NTN5239, the 2 channel controller is a NTN4862. According to the service manual, the NTN4862 was used only in models ending in 7120AN, but I don't think this is entirely accurate, so it is best just to open up the radio and have a look. The controller number is stamped on the flat side of the controller and is visible without any serious disassembly of the radio.
The conversion involves changing the selector switch from a 2 position toggle to a 6 position BCD and adding the 2 wires for the '2' and '4' positions. Then just program it with an archive from a 6 channel unit. The P100 RSS performs no serial number or model number check at all.
We would suggest obtaining the P100 service manual before attempting this upgrade. It will have the part numbers of the switch, knob and escutcheon needed. If your P100 is a low power unit (no Hi/Lo switch), use the old freq switch to add the Hi-Lo function. All P100's are built with the High power final Amp anyway!
The service manual shows a different part number for the top cover for a 6ch vs. a 2, but the only actual difference is the diameter of the hole for the freq. switch, so just drill it out a bit to mount the rotary switch.
General Information
A bad voltage regulator chip in radio causes a howl or microfonic RX.
There are three separate sources of five volts coming out of the regulator IC. Remove the bad regulator IC. It is best to cut the shield loose and break the IC loose by flexing the IC and the leads will break off.
Tie the three five volt lines together at the IC pads, and install a small three lead five volt regulator IC.
These radios can also have severe frequency drift due to the zener diode that regulates the 6.8 volts at this reference oscillator.