It’s kind of tedious, but you can do it with a tool like the DMT Diafold Serrated Knife Sharpener. On most serrated knives, you will also want to lightly sharped/deburr the other face at an extremely shallow angle using a whetstone that either you don’t care about too much or that is extremely resistant to scratching (e.g. a DMT-style flat stone). If you do that part on something like a traditional Japanese stone, you will make a mess.

I guess it depends on the style of serrated edge. But if you look at commercial tomato knives you'll see they're all serrated.

Is that just because they not expected to be sharpened though?

I don't think so, not at home.