"To add the stock" means to include or incorporate more of a given product, typically referring to a business needing to acquire more items for sale or supply. Full definition
Add the cauliflower and apples to the pot and level evenly across the pot, then add stock until it almost (but does not) cover the cauliflower and apples. (soupaddict.com)
Continue adding stock 3/4 cup at a time until rice is tender and creamy, 25 - 30 minutes total. (giveitsomethyme.com)
Gradually add stock until moist, but not mushy, stirring gently while adding, to avoid breaking up cornbread chunks. (thetasteplace.com)