Can I use a grass seed on an existing lawn that has some crabgrass and Bermuda grass problems, to help thicken up the lawn to prevent crab/Bermuda grass from growing? I live in Sacramento, Ca.
Reseeding is for thin, old lawns, not for existing bermudagrass/crabgrass control. Reseeding can help prevent these weeds, but when they are actively growing, they will outcompete the new grass seedlings.
To completely control Bermuda grass, you will need to get completely rid of it before you seed. Bermuda grass is a perennial, and the best non-chemical control method is tarping. Put thick, waterproof plastic tarps over the entire are, and leave in the hot sun for about 2-4 weeks (in my experience), maybe more. You will need to completely reseed the entire area. Digging doesn't work well. If you don't mind using chemicals, Bayer Advanced Bermudagrass Control for Lawns is selective, and does not harm bluegrass, ryegrass, or fescue lawns.
Crab grass, as an annual, is only difficult to control because it reseeds very efficiently. You can control it with overseeding somewhat, if you get the new grass established as far as possible during the dormant period of the crab grass. Crabgrass likes bare spots, so if these are minimized, crabgrass germination is much lower. If you don't mind using chemicals, Bayer Advanced All-In-One Lawn Weed & Crabgrass Killer won't harm your lawn.