Users can add a new variable following the the guide, which details the step-by-step procedure and includes examples using an example dataset also provided.
https://dtcenter.org/sites/default/files/community-code/add-new-upp-variable.pdf
Users may add a variable from their WRF output to pass through for output. They can also calculate a new variable, provided all necessary variables are read into UPP. The calculation may be done in the main routine for that variable (step 6 in the guide), or as a call to a new routine that does the calculation.