You can modify the source code a little bit. Look in the file,
linuxtv_muxes.h, easiest thing to do would be modify an existing
city to reflect the frequencies in your area, Say change Boston or
Philly for your local frequencies, then recompile it.
Your customized freqs will then be available.
If ya need more help than that please let me know...
I was unable to add a manual multiplex also, while setting up
a North American ATSC Frontend, Samsung 's5h1409' is my frontend.
Sounds like a user using an ATSC frontend cannot add a new mux manually.