I ran 2" conduit underground from bldg.1 to bldg.2 w/ 8' span between bldgs. Ran 3 10-2 w/g, using 2 of those for lights and outlets, the other one is for future use. It runs from breaker panel to junction box. Do I wire nut the ends and leave them in the box and panel until I use them or what? If a neutral wire is about 8" short can it be spliced in the breaker panel and with what or what is legal and feezible? Thanks, you guys are the Greatest. HAPPY NEW YEAR

Yes capped wires at both ends with wirenuts and leave inside boxes with tags identifying as spares and where the other ends are.

Yes you can splice and extend a wire in a panel.