How to make Grape Must
Based on the wikipedia article, it sounds like you want the pulp and skin as well.  It sounds like you won't get the same texture, consistency or sweetness by only reducing grape juice.

However, I found this cool how-to page on how to make grape must jelly.  I think the hand-crushing grapes part will be the most relevant to you.

You could use some kind of pressing machine, such as an OJ squeezer, to get the same effect without red hands.  I'd stay away from blenders and puree machines, as cutting foods doesn't have the same effect as crushing them.

I don't think I've had grape must, so you'll have to be the judge on how much or how little of the pulp goes into your final product.