You can not "ADD" memory to a video card.
Some of the older ones with sockets were upgradeable in the early to mi 90's. But these only had 1-2 mb of ram upgradeable to 4 mb.
A few years back we had "HYPER MEMORY cards that had 32-128mb of ram on the cards and then you shared system ram to them. But these were slow entry level cards and not really "GAMING" cards.