Well you could re-house (you wouldn't get the extra bits into a hand held) the unit and add an ssb generator for the transmitter and the ssb demodulator for the receiver. There's a phase type SSB generator schematic at
http://www.freeinfosociety.com/electronics/schemview.php?id=1917 SSB generation using the phasing method is cheaper than the filter method as you don't have the cost of the filter which can be $25 upwards depending on how good it is.
You'd then have to modify the transmit section so it operated in Class A so it was linear which is a requirement for SSB. If you are lucky then your RF transmit section will be linear already as in the Midland 507.
http://www.freeinfosociety.com/electronics/schematics/radio/midlandalan507.pdfSome modification would be required to the synthesizer to move the frequency range to where most CB SSB activity takes place and add an RIT control. Some CB synthesizer chips will allow this and it works well upwards in frequency for 10M FM conversions for amateur use. Unclear about moving down to 26MHz as I've never seen it tried.
OK I have to get un-positive now, to be perfectly honest it would be uneconomic to convert your rig and for what it would cost you to modify it (with no guarantee of success if you have the wrong type of synth chip) you could get one of any number of second hand multimode (AM/SSB/FM) rigs at a hamfest, garage or trunk sales for the same price or less than an SSB filter.
