Can I play cards into another players zone like the Orcs Souig Pen?

Also, can I place attachments on another players cards?

You cannot play cards into another player's zones. The two exceptions to this are when the card text explicitly allows it, and cards that are attached to other cards. The text of a card that can be attached to another card states what it can be attached to. For example, if it just says "attach to target unit", it can be attached to any unit in play, no matter who controls it. You still control the attachment though, even if you attach it to a card controlled by another player.