Well, if you're going to dynamically allocate the array, you will probably have to allocate a second array. This is necessary because your string s only has a fixed amount of memory allocated.
So, instead of tryig to overwrite the characters in your for loop, I would suggest incrementing a counter that told you how big your new array has to be. Your counter should start off as the size of your original string and increment by 4 each time an instance of 'b' is found. You should then be able to write a function that appropriately copies the modified string over to a new char buffer of size[counter], inserting 5 c's every time a 'b' is being found.
