There are people who use USB to serial adapters, some with limited success.
A lot of people use the Edgeport 8 USB to eight serial port adapter. You can find posts here and on the HomeSeer forum as to its success.
Look at
THIS post over at HomeSeer land, especially my last one for details on the driver for this unit.
Also, wait till other people respond with their success rates with other hardware!
On your other question, I actually like my CM11a and have it running on my HomeSeer server. I also have a Caddx Security System, NetCallerID, PH Anderson Analog to Digital Converter, and an Ocelot connected to my server via serial ports.
I did have a problem with some units not wanting to work on the upper comm ports, but I think it was my particular setup and not the result of the Edgeport 8. Another problem I had was the "hs.opencommportex" wouldn't seem to work with the upper comm port numbers as well.
When I installed the edgeport8 the serial ports were assigned as follows
Com1 & Com 2 were on the motherboard (original ports).
Then the edgeport skipped to Com 5 and started numbering the remaining ports from there (Com6, Com7, etc...). I had to use an upper serial port number for my PH Anderson Analog to Digital board, but "used" Com 3's resources to get this command to work (you would have to look at HomeSeer's command details for further explanation).