is it possible to keep my CV level up while doing gaining weight? (not too bothered about if my bf% goes up.)
while I would say you likely can't gain weight and keep in shape for a marathon I doubt thats your goal.
Just keep training cardio or sport specific goals moderatly while eating a shit ton and lifting right and I don't see how it would be a problem. Tons of athletes get bigger/faster and all around better in the off season. Just make sure what ever cardio you are doing is specific to your goals