Sorry - there is no "add this field to this field" operator in Web Screen Painter.
You'll need to write some simple SPL code which will take the values of the variables and add them together.
The easy part is the addition - you can use the standard Arithmetic Operators used here:
Where can I find Spel functions documentation?
CAUTION: This will involve going into the Source tab, which will move this into being a site supported customisation.
You will need to perform your addition and assign the result to a variable name, and then display that variable. Optionally you can create a custom form field on the table so that the value is retained.
Here is an unrelated code customisation query, but it will give you a taste for what can be done with a function.
Calculated Column in a list form? (12.6)
Does someone else have a simpler way of explaining this?