Or, heck, just do it during the events of #31. You know, the book where Jake's family had to go to his grandfather's funeral, and Tom's Yeerk got so afraid that it was going to starve that he tried to murder the rest of his family? A lot of people have wondered about that book, why didn't Jake just kidnap Tom then? It'd solve more than one problem at once.
Side note: that was also the book right after the one where we find out that portable Kandrona generators exist. So it wasn't like the Animorphs were the only ones being morons that time.
Anyway, as far as using Tom as a double-agent goes . . . yes, it makes a lot of tactical sense, but I can't picture Jake actually agreeing to that. If he ever did actually manage to free his brother, I think he would be desperate to keep him as safe and as far away from the Yeerks as possible. I don't think he'd be able to stand the thought of losing him to the enemy a second time, this time as a result of his own actions. Now, I can picture
Tom being the one to argue to go back to the Yeerk pool as a double agent. The one and only time in the series we see Tom act as a free human being, he runs straight at Visser Three armed with only his fists, giving up his one chance at freedom to help the 'Andalite Bandits' escape the Yeerk pool. That's a guy who would be more than willing to risk his life for the cause. If Jake would allow him to do so, that is.
Although, it's also worth noting that this whole plan would be a HUGE security risk. Suppose they tried to recruit the wrong Yeerk to the cause, and that Yeerk blew his cover to Visser Three? He'd have an Animorph in his head, sure, but could that Animorph manage to morph and demorph in time to keep Tom from being infested by a different Yeerk, or killed? Keep in mind, if they infest Tom at that point, that's pretty much game over.
Safest move is to get a random Chee to fake their current identity's death, and then take over as Tom 24/7. If you're still set on the double-agent plan, maybe recruit one of the free Hork-bajir. They'd be less suspect as double-agents because of their low intelligence (I feel like Yeerks wouldn't really take them seriously as a threat), and if caught, they'd be better able to defend themselves than a human would, giving the Animorph-turned-Yeerk at least a little more time to demorph and remorph before getting reinfested or killed.
. . . Actually, you'd really only be risking them getting killed. A Hork-bajir could slit their own throat if they were about to be infested. Which, yeah, would be sad, but at least the Animorphs' identities would still be safe.