Are you asking if you can start a new game, but take Squirtle as your starter Pokemon?
If so, then sadly you can't. I would've loved to play through HeartGold with a Bulbasaur as a starter. :(
If you're able to, you could trade your Squirtle over to a different game (which would require 2 DS' or a Wi-Fi connection and a trustworthy friend), and then trade back to your SoulSilver game.