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Technically speaking there is no such thing as a DC transformer of anykind be it step up or step down. Transformers are strickly AC. There is probably some kind of electronic device that could take the DC, change it to AC, step it up with a transformer, and then change it back to DC.
"Kioolt" is absolutely right. If you connect a transformer to DC, it'll turn into a smoke bomb right before your very eyes. The devices that convert DC votage to a higher DC voltage are often referred to as inverters (although that term usually applies to "DC to AC" units). There are units available that do what you want. They would either work like "kioolt" said (DC to AC, step up, & back to DC), or they would ELECTRONICALLY increase the DC voltage. If you do some "Electronics" searches on the web, you'll find some outfits that make such units. I've seen a few around.
You can buy voltage step up power supplies for different inputs and outputs. They tend to be kind of pricy and don't put out a lot of current. More current equals higher cost. Why not convert the 6v syatem to 12 v?