I'm adding a circuit in my basement. The wires run to the right of my panel, but I'm out of slots on the right side of my panel. Can I tap into the box on the right, and run the hot wire to a breaker on the left? Or do I need to run the cable around the outside of the box and tap into the left side so the wires don't cross inside the panel?
Yes, of course you can. The ample margins on all sides between the breaker area and the wall of the panel is considered a type of raceway, and is precisely for that purpose.
What you cannot do is route conductors across the area where breakers are supposed to go.
There are those who say you should nip the wires back smartly so the panel is very neat. That is nonsense. Leave yourself enough length that you can move breakers around if you need to. While some consider the extra wire length to be ugly, what I consider ugly is yellow wire-nuts all over the place because someone cut wires too short.
Do not coil extra wire (you don't need that much extra) or you will accidentally create an electromagnet.
Makes no difference where and cable enters the panel and once it is there it can take power from where ever there is an open space.