how can I access the electrical panel box
Yes, you can use sides, bottom, back, or even make your own hole (within reason).
All of them have knockouts (though the back tends to be fairly sparse).
If you come through a joist, you may have trouble attaching a cable clamp.  I would install a metal box (say: 6x6) on the other side of the joist, use a Rigid short pipe (conduit nipple) through the joist with about 1/2&quot; of thread exposed on both sides, and clamp it down hard with conduit nuts. Then I'd bring a ground wire through to ground the metal box, and bring my cables into that box. Tie the grounds to the box, and tie the hot-neutral to THHN wires to extend into the panel.