What do you mean by "external means"?
No, you can't add dedicated video memory. Yes, you may be able to change the size of the shared memory (in BIOS setup). Why would you want to do this, escapes me though. Giving away system RAM for almost nonexistent increase in video performance. Going over 8MB of video RAM is useful only if you have a monster card that can do true 3D or if you plan on using video resolutions over 1600x1200.