Well yes, in fact you probably can. Though it’d take a lot more experimentation (getting the right amount of gelatin in your juice and such). The great thing about JELL-O as I said is that it’s pre-formulated to a firmness that’s perfect for these sorts of shenanigans. Gelatin you see, comes in many different consistencies. It’s degree of firmness is measured in what’s called Bloom value, a low bloom value being very firm (for things like gummy worms) and a high Bloom value being rather squishy (aspics, jam and things of that nature). JELL-O is relatively weak as confectionary gelatins go, but not so weak that it won’t hold it’s shape in block or cylinder form. I find that granulated gelatin is a bit too soft for those sort of applications. Yes, you could just add more, but lots and lots of gelatin makes for a gummy, rather than rigid, consistency. Better to use stonger gelatin in smaller amounts.
Oh, and to make one of these just mix up two or more of your favorite flavors (I like lemon and lime) and pour them alternately into a glass, letting the lower layers chill for about half an hour in the freezer between applications. Be gentle. For this I used a turkey baster to dribble the top layers in. As they layers chill, simply hold the different mixes at room temperature. JELL-O doesn’t gel until it hits the fridge.