They are already bonded to the spren who follow them around, making this possible. Scary thought: the Everstorm making stormspren bond with chasmfiends, and storm form chasmfiends all over the shattered plains. Prob wouldn't work like that.
I just thought of another theory: on Scadrial, Ruin is able to control people who have been pierced by metal, and people bonded with stormspren are controlled by Odium. What if this also works for whatever spren chasmfiends bond to? Those allow them to grow to such size, and looking at the Greatshells in the Reshi Sea, it looks like they could be related in some way to Cultivation. The prelude specifically mentions red, orange, and violet blood. Three types. Red for men, fighting for Homor, Orange for the forces of Odium... Purple for the chasmfiends fighting for... Cultivation, controlled by their spren bond?