Can a licensed ham use or modify CB equipment to work the 10 Meter Amateur bands? If so, are there any advantages/disadvantages to this?
You can, but you probably don't want to.
CB radios are limited to a set of 40 channels, most don't have frequency control. That somewhat limits the utility in the ham band where we have a very wide swath of frequency we can use. A converted CB radio would, at best, have control over only a narrow portion of the 10 meter band.
They are also limited in power. While external amplifiers for CB are available, they are illegal, and would not work well on the ham band anyway. If you wanted to operate at higher powers than those allowed by CB, you would need a ham amplifier.
Most are also limited to AM operation. Most ham operations on 10 meters are SSB. You would not be able to join in most conversations, and most users would think your transmissions were either garbled or interference.
Simply put, for the technical expertise, time, and parts required, the value just isn't worth it, when you can get a multi-band HF radio on eBay for a few hundred bucks.
To extend on the legal part of the answer:
Hams can use any radio to transmit, as long as they are in the right frequency band, are transmitting using allowed power, etc.
Non-licensed radio services on the other hand are subject to a host of restrictions, generally including fixed antennas, small power limits, radio-by-radio certification, etc.
To put this another way, the FCC trusts you to keep your equipment legal with ham radio; for no-license services the FCC trusts the equipment.
You can use whatever you want as long as it legal. No white lists or certification.
That being said, it would probably be a pain.
There are some CB-style "11m" multimode (AM/FM/SSB and sometimes even CW) sets that operate in bands at the top of 27MHz, and some of these can be made to work at the bottom of the 10-metre band.
These can usually be used legally by licensed amateur operators, as long as they stick to frequencies for which they are licensed, and follow the band plans for that portion of the band.
This is true for the US and European countries. However, it is NOT the case in all countries.
Some countries only allow the import, possession, and use of specific models of radio transmitters/transceivers, and using any other equipment can result in a fine, confiscation of the equipment, and even prison. An example of such a country is Thailand, where I live and am licensed (with a reciprocal licence - 'real' licences are for Thai citizens only). We have to import radios, and have them checked by the Broadcasting regulator before they will issue a licence for the equipment. We also need a licence to operate (the actual radio licence), and a licence for the premises where we will operate. The list of equipment that can be applied for approval does not include any radio equipment with 50MHz capability, so all modern HF sets are not allowed. I use an Icom IC-718, one of the only HF-only sets still sold.
However, despite these restrictions, Thailand still manages to have the third-highest number of licensed radio amateurs of any country in the world (behind Japan and the United States of America). Go figure.
Yes, you can convert a CB to a ham band radio. Even an AM radio can be used in the ham bands; a small portion of the 10 meter band is allocated to AM service (29,000,000 Hz to 29,200,000 Hz).
You don't need an amplifier. Many hams work varied countries using a QRP rig. Most QRP rigs are around 5 watts or less. I've heard a few say 10 watts or less is QRP, but on average it is 5 or less, so an unmodified CB is considered QRP. Anyway, anyone who is anyone in radio knows it's not critical to have an amp, when you get the most bang for the buck with a good directional or high gain antenna, which isn't as hard to do at these frequencies than the low bands, due to antenna size.
But it takes good engineering techniques to get a CB radio to shift that far from its normal range of operation. Many of the the techs I've talked to say some won't track that far because the VCOs don't like the voltages it takes to go that far and the PLLs used in them don't like it either. So be set up with the equipment to test your mods, till you get it to do what you're attempting to achieve.
Yes, you can. If you are an amateur radio operator you can grab any radio including an American / Australian CB and use it as long as it has been modified to operate on the amateur bands. Australian / American 27 MHz. CB's were easy to modify to 28 MHz. Both crystal synthesis & PLL, However, I have only known of one that was modified to work on 21 MHz.