I'd guess it's possible using the postflight script, but I'd need to look at the Dock files (using Windows at the mo) to see how. Possibly via AppleScript?
You could use the postflight to launch a small application/applescript to prompt the user, but it might be a bit of a kludge of a design.
You're probably better off just showing the application in the Finder once it's installed, hence the user is likely to launch it. Once it's launched, you can provide a dialog on first launch to ask the user if they'd like the app to remain in the Dock.