i know that when someone goes insane you have to destory all items attached to someone but after they are already insane can you then choose to attach something to them?

Yes as long as it doesn't have any restrictions as you have to keep in mind that an insane character doesn't have any traits, faction, textbox, etc...

So you can't put a prize pistol on an insane character because its not currently an Agency Character for example but you could put Infernal Obession on since it only says a non-ancient one and sinces insane characters don't have any traits/subtypes you can steal it.