Here is their index on the topic:
STEM CELLS
meaning of term: g02 11/22 4
medical usage: g02 11/22 3-9
Bible view: g02 11/22 9-10
They would reject based on certain sources and delivery methods. For example, embryologically derived stem cells would be rejected. However, I could see them accepting induced pluripotent adult cells - if not derived from blood, but say, skin.