It's not impossible, but I feel like it's unlikely.
For one thing, Imu's just too short to be in Joyboy's body. From the one time we've seen Joyboy in Emet's flashback, he seems to be more or less standard human size. Scale is unreliable at best in One Piece. But, as best as I can tell, Imu is actually pretty short. Like...young child or old person size. Imu looks shorter than the swords that surround the Empty Throne. Unless Joyboy's body has significantly shrunk over the course of centuries. Which isn't impossible in One Piece, given how we've seen people age like Gloriosa or Hyogoro. But, unless we assume that Imu has let this body age and never got the immortality surgery, I don't see it happening. I don't see the point in Imu being in Joyboy's body, if Joyboy's body has gotten super old and shrunken down.
For another, it'd be weird for Imu to be afraid of Joyboy's Haki Knot, if Imu was in Joyboy's body. Unless we assume the reason for Imu's fear is that Joyboy's Haki could push their soul out of Joyboy's body? That's possible. I'm not sure where Imu's soul would go in that situation. Would Imu's original body still exist? But, it'd just be weird for Imu to be okay with living inside the body of the person they seem to fear the most. If Imu's inside Joyboy's body, Imu should have access to Joyboy's Haki, too. I'm not sure if it makes total sense to be afraid of Joyboy's Haki in that situation.
Third, that would have to mean that Joyboy never had the Nika Fruit. Since Luffy has the Nika Fruit, it's previous user had to have died. So, if Joyboy's body never died, that means any Devil Fruit he had would have never reincarnated. Now, no one's ever said Joyboy actually had the Nika Fruit, so that's not the biggest problem. But, it does seem likely that Joyboy should have had the Nika Fruit, in order to better parallel Luffy. So, if you think Joyboy had the Nika Fruit, then that means that body cannot still be alive, because otherwise the Nika Fruit would never have reincarnated and Luffy never would have gotten ahold of it. Again, Joyboy doesn't NEED to have ever had the Nika Fruit. But, it would be a little unexpected if he didn't.
Finally, we've seen Imu transform into a "Yokai" form, that kind of resembles some sort of a dragon. Would that mean that Joyboy had a Dragon Devil Fruit? Again, nothing has ever said he didn't. But, it would be odd for Joyboy to have a Dragon Devil Fruit, rather than the Nika Fruit. Not impossible. But, it would be kind of a unexpected twist.
So, I don't hate the idea. I just don't agree with it. And it's possible that Imu does have Joyboy's body somewhere. Maybe even in one of the freezers below Mariejois, to go along with that Giant Strawhat? But, I don't think Imu's soul is inside Joyboy's body. I don't feel like it quite lines up with what we know.
