Do I need to add some 6-Row or otherwise tweak this recipe? I'm going to be brewing a dunkelweizen later this month. My recipe uses 6 lbs. of wheat extract and 2.5 lbs. of caramel and chocolate wheat, and the yeast is Wyeast's Weihenstephaner (3068). I'm planning to steep the wheat in a mesh bag in 5 gallons of water for at 155F for 45 minutes before the boil. Looking at some all-grain recipes for dunkelweizen I see that some add 6-Row barley to help the wheat convert. Do I need to add some to my recipe or will the liquor I get from steeping + the extract ferment well enough on its own?
If the reason for doing so is enzymatic conversion, then no. Malt extract, wheat or otherwise, has already been mashed, and thus already had it's starches converted to sugars, and will ferment just fine.
The reason for doing this in an all-grain batch is because wheat doesn't have enough enzymes in it to self-convert its starches into sugars during the mash. 6-row has a high enzymatic content, and thus will have 'extra' to convert the wheat starches. This is one reason why you'll see a base barley malt, either 2-row or 6-row, in any wheat beer all-grain recipe.
The caramel and chocolate wheat are there for flavor additions and don't really need to be converted. The caramel has already been effectively 'mashed' during the process to make that type of malt, so there aren't any starches to convert.
Wheat malt is easily able to convert itself without further diastatic power form other malts. Unmalted wheat does need help, though. And these days, 2 row malt has nearly the same diastatic power as 6 row. The notion that 6 row is necessary to help other malts convert is outdated. The reason you see barley malt in most wheat beer recipes is for flavor, not enzymes. An all wheat beer is very bland and insipid.