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Yes, you can define multiple stock bodies:

Example:
If you have four bodies that you want to define as stock you will have to initially select two of the bodies to do a Union. You can then add the third body to the two bodies you have just designated as stock, then the fourth body. You can only add bodies one at a time.
1. Go into the Solid Modeling Pallet
2. Select two bodies 
3. On the Solid Modeling Pallet, click Union.

A Union will be applied to the two selected bodies, you can keep adding a body to create more than 2 pieces of stock.

Right clicking on the body and going to properties allows you to choose either, Part, Fixture or Stock.