hey William... i'd imagine any audio files you have on the HD5 is stuck in there. IIRC Sonic Stage is like the roach motel. audio files go into the Network Walkman but can't come back out.
as for the K750, if it is anything like my W800 (and i'm sure it is), if you loaded up all the drivers and software for the phone on your computer, connect the phone with the USB cable, and it should appear as a removable drive in your Windows Explorer. then just drag and drop your mp3s to that drive, but make sure your mp3s are nestled in these folders, or else the K750's music player will not be able to play them:
J:\MSSEMC\Media files\audio\Primal Scream\Screamadelica\Higher Than The Sun.mp3
of course drive J is just an example, could be different on your computer, and you might not want to listen to Bobby Gillespie's masterpiece on your K750, i just have that for example's sake.
hope this helps!
oh, and yes, you should get a card reader instead... much faster than through the phone.