You could try editing the database tables directly: wp_popularpostsdata
holds the “All-time” Time Range info, and wp_popularpostssummary
contains the data of the rest of the Time Ranges. The easiest one to edit is the former, the latter stores daily views data and represents a lot of manual work if your site has been running for a while now.
I did plan on adding some sort of data import to retrieve data from Google Analytics a couple of years ago but the whole process was so complicated in terms of user friendliness back then that I discarded the idea completely: the steps required to integrate WP with their API is not something an average user -eg. a graphic designer, a blogger or a shop owner- could do easily, which goes against my it-has-to-be-simple philosophy.
I do see the value of being able to import data into the plugin, it’s just that if I decide to add such a feature I have to plan it out well so everyone can do it painlessly (which in the end would make my life easier as well, less chances of getting “it doesn’t work” / “I don’t understand how to do this” reports.)