Do you mean to the California Grill restaurant?
Technically it is possible because they do permit guests to access the lounge without a reservation.
However, they will not allow this if the lounge is full. You would have to check at the second floor desk for the California Grill and have them determine if there is space for you to go up to the restaurant. Around fireworks time is usually when they stop allowing access for those without reservations. You could likely pull a lounge seat for fireworks off only if you go much earlier than the fireworks show and plan to stay awhile. You can order from the full menu in the lounge and bar, if that interests you.
I don't know if I would suggest you go straight over to the Contemporary from DHS around time for Wishes with the intention of watching Wishes from the lounge at California Grill. You would most likely need to get there well before Wishes starts, or you may be turned away at the second floor desk. I'd suggest you arrive there no later than two hours prior to the show, an hour and a half at most.