El Capitan app/extn to allow whole-screen zoom with just trackpad
I use BetterTouchTool (BTT) for customizing a trackpad (or I did before my trackpad fell apart has yet to be replaced). I know that BTT can do what you are asking. Since it can also be used to customize most other input devices (keyboard, mouse, remote, etc.) and not just a trackpad I still make use of it without a trackpad and with or without I consider it one of my essential pieces of software. I've moved most of the my shortcuts to BTT so that all my alterations are in one place.

BTT allows you to set up most any gesture you'd like, not just the standard Apple gestures. The downside is that the documentation is somewhat sparse and it can help to have a certain amount of technical experience to figure out how to set up certain functions but your desired need shouldn't be too difficult to configure. 

I believe that you can download a trial version before deciding to pay for the app and the license cost is very reasonable and done as  a "pay what you can" sale (with a low minimum cost). (BTW, I'm only a user  of BTT and am not connected with it's development or sales.)

BetterTouchTool is available from BetterTouchTool.com which actually redirects to the developer's site: https://www.boastr.net/