How do I add another option to an existing form?

I have created a form for our members to join our organization.  Right now they select the type and term of membership and it has a set price.  I'd like to add a donation option to the form.  Is this possible?  It would look like this.

Additional donation to O.T.A. Develpment Fund  (please specify amount)  $0.00

From what I understand it seems to me that you want to allow your form to include donations.

There are a couple ways to do this but, it mostly depends on how you want it to function.

Do you want that to be it's own option or is it going to be included in the sum with your membership pricing setup?

You can either keep them separate by turning your form into a multi-payment form.

Alternatively, you could change your products over to a field then add another field for donations which could use a form calculation to generate the sum to be pulled into your payment integration if you wanted it to be inclusive all on the same form.