I sometimes buy FLAC downloads and transfer them onto the B2. I put them on a USB stick or transfer via NAS from my laptop, as Daniel describes. I've never had to reformat into the artist/album/tracks format, the album art is always included and has never failed to transfer to the B2 without any tweaking. I've mainly bought from HD Tracks. Albums usually arrive as zip files, but I've managed to sort that out, despite not being much of a techie.
One thing to say, though, in case you're not aware, you shouldn't try to add anything higher than CD quality to the B2. Firstly it may not play (though at least some 24/96 files will) and, secondly, there's no benefit. Even if a file plays, the B2 will cap playback at 16/44.1 (CD quality) so you're wasting money and disk space on anything higher if the intention is only to put the tracks on the B2. I've converted my higher res files using dBpoweramp software for B2 use. It's a FLAC to FLAC conversion, but you can alter the bit depth and sample rate within that.