Is there a way to add a byte to an arbitrary point in an appvar, without overwriting any data. Would I have to push all of the data 1 forward? I looked though the fileioc docs and couldn't find anything. I am just hoping there's an easier way before I make any rash decisions. Thanks.

Short answer: no

Any singular file you store in flash is going to be contiguous, including appvars. This means that all the bytes in the file are stored in order from the start of the file to the end of the file. This is what allows the use of pointers. If you, for instance, have an array of sixteen two-byte values stored at some point in an appvar, you can iterate through them by using the pointer (memory address) of the start of the data and adding 2 to it to locate the next value. If you "added a byte to an arbitrary point" that would require moving everything after it down 1 byte, which would ruin all your pointers.

In any case, I'm confused as to why exactly you would need to do this. If you let us know what you're trying to do, we may be able to help.