connecting new circuits to outdoor circuit breaker box

hi, my circuit breaker panel is on the outside of my house and there are 5 different sized conduits ranging from 1/2 inch to 1 inch that are used to house wires to feed the basement. i want to clean this up and have only one large conduit or maybe 2 that house all the wires. right now they run romex still in the sheath up the conduit to the box. my question is, can i disconnect some of these wires and pull them through a larger - like 2 or 2 1/2 inch conduit? do i need to strip the sheath off the romex so i have more room? i know that code today requires THHN wires from the box through the conduit to the inside of the house, but i do not want to put a large junction box down there. i just want to get rid of some of those conduits and consolidate my wires in one or two conduits. BUT! if i do have to use a junction box, can i put a grounding bar in there and a neutral bar in there and only run one white and one ground in the conduit?

thanks! i'm heading to work over there today and will take some measurements and photos