DSLRs typically have an HDMI port allowing you to output the live view of the screen at 1080p over HDMI. With a "capture device" you can then use this HDMI stream as a "webcam" to record in Screenflick. There are many of these devices, but basically any device which is described as "plug and play" and doesn't require a plugin/driver, will be the easiest solution.
Here are a few USB HDMI capture devices that should work:
The other option is to record directly from the camera over USB. This doesn't need any additional hardware, but the size of the captured video may be smaller than the usual 1920x1080 because of the transfer speed of slower USB 2.0 class devices. Newer cameras should allow for capturing higher resolution.
Canon has recently started providing the EOS Webcam Utility which allows Canon cameras to work in software all across macOS, including Screenflick. There are limitations due to both the hardware capability of the cameras and USB, as well as limitations in the software, but this is a most-welcomed new feature that Canon is providing.
A good article about the EOS Webcam Utility: Photo Taco
PTP Webcam is a plugin for macOS that allows a Nikon DSLR to be used as a camera for recording in Screenflick.