Does not appear to be expandable. looking at the pictures online, the case doesn't even have a slot. Looks like all add-ins will be via eSATA and USB.
IT does state there is room for a second 2.5" hard drive. It will also support an mSATA card. (a SSD hard drive on a special board)
You may have to look for a USB digital tv tuner (and one that supports comcasts encryption, if one even exists) You might want to ask comcast what they have available. The solution will be similar to a laptop solution if the comcast support goes blank when asking about a VISION HT.
I quickly read that some ClearQAM (unencrypted) tuners might work for some basic channels, but i have also read that those channels are disappearing quickly.
Looks like those Ceton guys make a USB variant of what you posted above. here
I cannot vouch for its value, or if it'll even work with comcast (as most all providers are moving to encrypted QAM channels and require specialized hardware)
I assume you have done your homework and found out if it works with comcast or not.