Even though your sites are already hosted at DreamHost, I don't think this will simplify the process.
Basically, you will need to follow the same process as if your sites were hosted elsewhere;
Move any domains across to the new account.
Backup and copy all site files.
Backup any databases used by the sites.
Re-Create the databases under the new account.
Restore the databases.
If there is an easier method, I am unaware of it.
Save [color=#CC0000][b]$50[/b][/color] on DreamHost hosting using promo code [color=#CC0000][b]SAVEMONEY[/b][/color] ( Click for promo code details )