In short, it's not possible at the moment.
The icons.tbl is almost completely hardcoded, in that you can only control (via tables) which entry links to which effects file. There are a hardcoded number of entries that the game will allow. These are the ones you see in the briefing editor in FRED (Gas Miner, Knossos Device, Planet, Asteroid etc.). Each entry points to an effects file (well, files, that is Fade, Bitmap and Highlight) which is what you see in-game.
What's more all of these entries have some specific traits, for example Unknown will always show the 3D-question mark when clicked, Asteroids will ways show the Asteroid ship type when clicked, and the planet can't be clicked at all no matter what you do with the table.
Anyway the point is, you can't add any new entries, only modify existing ones. Fortunately a lot of them are duplicates. You could probably swap Largeship and Largeship Wing (which point to the same effects as Cruiser and Cruiser Wing) off for your own icons. Any other built-in icons you never use such as Support Ship or Transport Wing can also be replaced.
I think this has been changed in newer builds, or at least will be changed soon. For now you'll have to settle with modifying the given ones unless you can find a build with a dynamic icons.tbl. Which AFAIK doesn't exist. Yet.
