You did not specify your Sony TV model but chances are that the dot (.) command is already there. Check the "Learn infrared commands" and the "Customize the Device, Sound, and Picture commands for this TV" (both under Device > More Options). Try it in device mode and if it works, then just map it to a key in the Activity. Some people map it to the +/Clear key. If it does not work, then teach it from you original remote using the "Learn infrared commands". Just select it and click Learn. It will be the first one in the "Other Commands" section (just above the "10+" entry).

Currently there is a bug with the "Favorite Channel" for the 6xx series (don't know if the 880 has the same bug). On the 6xx, the "Favorite Channel" interface will accept the dot and the dash but you get an ASSERTION error during the update process.

The Activity Start "Set it to this channel" feature does not accept a dot (.) nor a dash (-) in the channel number. The workaround is to simply use a sequence of commands (via the "Send this IR command").