I would not substitute anything for what the instructions call for, it will change the diet and all of the work behind it to ensure the right amount of nutrients are being offered.
If you are asking about a one-time or temporary diet until you can find or re-purchase new supplements, then yes. You can offer temporary other sources of protein and fruit/vegetables. The protein source can be boiled or cooked, unseasoned chicken, hard boiled or scrambled, unseasoned eggs, and you can offer yogurt, make sure it does not contain aspartame.
I wouldn't use this temporary diet long, but it would do for a little while, the sooner you have them back on their bml, the better, in my opinion.