Not at this time. Unfortunately, there are countless bug and issue trackers out there and just as many different formats for the data. Moreover, Sifter is simpler than 99% of them, so we simply don't have the fields to accommodate all of the data that other systems track. However, we are working on creating an API that will hook you up with the necessary tools to import things the way that works best for you.
For now, if you really need to import issues, our current recommendation is to use our email integration. It's not quite as fully functional as the write-API will eventually be, but it's ready and available today and is surprisingly flexible for getting issues into Sifter.