What do you mean by strong? Hot alcohol? Dry? Smooth but warming? Sugar will bring it back from dryness but won't change alcohol warmth. You have to sulfite it before adding sugar or you can use a non fermentable wine conditioner.
Time will usually allow the alcohol 'taste' to mellow as the other flavors blossom. If it's really too extreme for your tastes and/or don't want to age, try blending with another wine.