Is it possible to use a 3rd party software (such as Bundler ot Agisoft) for initial processing such as finding key points and their correspondence, and then import that info into Pix4D and basically just use Pix4D for dense reconstruction? Which is what you can do in SURE software.
It is not possible to import into Pix4Dmapper the results of keypoints extraction and skip the first step of Processing (calibration, keypoints extraction, etc).
We really believe that the 1st step of processing gives very good results. We have several users who use the outputs of our software (optimized internal camera parameters, optimized external camera parameters, undistorted images, etc) in traditional photogrammetric software (mainly for stereo-viewing). An example here: 202556659.
We strongly recommend you to try the 1st step of Processing with Pix4Dmapper and compare the results.