A Not a good idea. The whole purpose of juicing is to gain the fresh raw nutrients and enzymes present in the fruits and vegetables. Cooking would greatly reduce the nutrient value and the overall benefits.
What you're actually describing is vegetable broth which does contain minerals, but we're after much more here on this raw foods site.
Juice them raw and as fresh as possible. If it's the taste of the greens you're worried about then juice some carrots or fruit along with them until your taste buds adjust.