You certainly can. If they were being raised by a broody she would be feeding them fruits and veggies at that age if they were available. But that broody would also make sure they had grit. Mary is right, grit is important.
CT also makes a very good point. The majority of their food should the regular chicken feed. It’s a balanced diet and they really don’t need anything else. If you feed them too much fruit or veggies it can upset that basic balance. How much is too much? A rule of thumb is if they can clean it up in 15 to 20 minutes, you ae doing fine.
I once gave one of my puppies a pigs ear - and she had the squirts all over the wall!!!
I gave my pullets watermelon this past summer and they got pretty runny and squirty too.
So, as with introducing new food to any creature, only introduce one new food at a time in small quantities.
And only introduce a new food every other day - that way you can gauge how well they respond to it.