Can I create a field at the bottom of a form that adds up the total of several number fields?

Do you mean to add values of different fields to a total at the end of the form? If so, you may use the Calculation widget in the form to perform calculations.

Please refer to the guide below that can help you with this.

http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form

As I check your JotForm, I see that the calculation widget is already added to your form to add the fields. If you are referring to something different or need any further assistance, please let us know. 

Thank you!