How do you form "ROMgI" from "RMgI" reacting with ketones?

Oh, you're referring to a Grignard Reagent variant. Normally you should see $- O - M g B r$ more often. I saw this in second-semester Organic Chemistry.
While the formation of $R - O - M g - I$ from $R - M g - I$ needs to be dry $- M g B r$ in the presence of water, the hydrolysis process of $R - O - M g - I$ is actually acid-catalyzed. Also, what's attached to the $- M g - I$ must be alkyl or aryl (R = alkane or 1-benzene derivative), at least for $R - M g - B r$.
Magnesium, on the other hand, is a strong Lewis acid, so it's a strong electron acceptor. As a result, $- M g I$ becomes a great electrophilic site, since nucleophiles are electron donors.