I don't mind black bars - how do I create 1080p video from DSLR stills?

I've got lots of time-lapse video made from DSLR stills.   DSLR's make images with a 3:2 aspect ratio, e.g., 8256x5504 pixels.   I want to put them up on services like Vimeo.  

If I put a conventional 1080p (1920x1080)  video up on Vimeo it's happy.   But if I put a 3:2 aspect ratio up (1620x1080) it black-bars the sides, as I would expect, and the top and bottom, which makes no sense!   Basically it shrinks the whole thing.  See below . . . The top image is a 1080p video.

I don't want to crop my images and lose content, instead I'd rather just have black bars on the sides.  I want to make my 1620x1080 video have black bars on the sides to fill it out to 1920x1080 so I can upload a conventional 1080p video to make them happy.   How do I do that?

I can't be the only person making time-lapse videos from stills, and there are tons of how-to and tutorials out there but I haven't found any that address these details.   Thanks in advance.