Which recipe are you trying to make? I don’t cook either, so I’m not sure how much help I’ll be…

I’m capable in the kitchen…. and even better when it comes to baking. I searched on rabbit.org and only one result – Brenda’s Homemade Bunny Biscuits.
http://www.rabbit.org/fun/biscuts.html

No mention of frozen berries, so I’m guessing not what you’re looking for.
Also a couple ways to hide meds in food recipes, but still nothing with berries. But, I’m guessing you could swap the berries for the banana in the above linked recipe, or at least partially. Banana is an often used egg substitute for people attempting to bake vegan, so it does help somewhat as a “stabilizer” of sorts, so you may get a better result to leave some of it in. Perhaps switching it out for the carrot would be a better idea, as both banana and carrot are high sugar content it should be palatable regardless of what you swap for and no real nutritional advantage either way. I would stick to replacing equal amount with an equal amount, or if you want to keep everything in the recipe but add berries… I would say make sure you thaw and drain them first as frozen is very high water content, add around 3 tbspns, and then up your pellet and oats by an about 2 tblspns each.

Okay, yeah, so it was the one I linked. I would advise swapping the berries for the carrot, 1/4 cup is probably reasonable, I wouldn’t do too much more than that without adjusting the other ingredients upwards. Juice wouldn’t really bind, if you want to leave the honey out I’d say a small bit of flour would do wonders. If you read carefully, the author mentions it used to be made with flour instead of ground oats. Oats are a healthier choice, and I love em personally, but flour forms gluten bonds when it contacts liquid (your berry juice) which really give the best structure of anything in baking, so if you’re concerned about it’s ability to hold itself in one piece swapping the tbsp honey for tbsp flour will be perfect!

Of course the honey does work, like it’s like granola bars, keeps it together but in a sticky floppy “glue” sort of way rather than a firmer structured cake/biscuit sort of way. I am not a bunny nutrition expert, but ultimately it’s carbs and simple sugars either way which should be limited for bunnies, so maybe you should just go with either flour or honey and base that decision on what texture you think your bun will like best. Or perhaps just what you have in the house!

Honestly the thing that I love about making treats for animals is they don’t complain. Just wing it-it will work. Trust me-the fire alarm goes off if I boil water. I am no kitchen genie. But you could substitute a carrot sized-ish amount of berries-and it will be fine I’m sure

Like was mentioned, I’d just be concerned about the berries ability to hold up in the treat. Carrot seems to last forever in treats, berries not so much…