Generally, you can’t have two different types of input in the same field. Furthermore, you won’t be able to run the formulas “only if the field is empty” since that will occur circular reference.
Can you allow the Ragic support staff access to your account so we can have a look at your current design? Also, please kindly provide an actual example of the formula you would like to have based on existing entries. Then, we can evaluate if there is any workaround for you.
You can allow access by clicking the “Need Help” link on the upper-right side of your account and filling the form.