You could Request Data Export from Garmin…
They’ll send you a zip file full of 
which will have a folder structure like:
DI_CONNECT/DI-Connect-Fitness-Uploaded-Files/UploadedFiles_0-_Part1
(with Part1 continuing in other folders like Part2 etc)…
Those UploadedFiles_0-_Part* folders will contain fit files that you can semi-bulk upload into Runkeeper (I think it’s 100 at a time)
There’s this idea of Manual Upload in Ideas that could use votes… it’s an old thread that came up at a time when I was really running up against Strava’s API limits, so a lot of the conversation in that thread is tied to that issue. I don’t have a good sense of the work to impact ratio on that 
so the vote count helps a lot.