Unfortunately can't make them spawn. Though it'd be pretty awesome if we could! They're a bit of a sore to farm since they spawn randomly in certain 3D missions. I've gotten the Ellie Fantone achievement after about 100 tries of farming her, whereas my Mikael Woltorf I got pretty much off the bat when I first started playing.

I will be grateful to finally finish my exec achieves. :sigh: Good luck if you decide to start farming them and patience is always a virtue!