I asked a really similar question yesterday and got a great answer from David. Here's the thread: https://trifacta.force.com/Customer/s/question/0D51L00004lcuOjSAI/can-i-add-a-prefix-to-data-in-a-column
If you tweak the formula described by David, you can surround the "merge" function in an "if" statement. So your formula would be something like: if(length(zipcode)<5, merge(['0', zipcode]), zipcode)
