Is it possible to add a donation request in addition to the product?

We are selling tickets to our conference, but we  would also like to add a donation option where the person can also input their own amount. Is this at all possible?

Unfortunately it is not possible to sell products and collect donation as well in the same payment question but with a little workaround, it is possible to achieve your requirement. Here are the steps you should follow:

#1. Instead of displaying the products options in the payment question, you can add the products in a check box question or whatever other field best suits the ticket options.

#2. Assign calculation values to those options:

https://www.jotform.com/help/301-How-to-Assign-Calculation-Value

#3. Add a text box or number field to collect the donation amount. 

#4. Add a "Form Calculation" widget in your form to calculate the total amount of selected product and donation amount from the text box.

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget

#5. Add desired payment question in your form and set it to read the amount from the calculation widget. The following guide should help you how to pass calculation to payment question:  http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field

This will allow for setting up multiple different fields of varying types then totaling up the values.