SD card for storage?

Sorry if this has been asked here before, I did a search but couldn't find anything.

I've recently set up a Pi with HifiBerry Digi + Pro, running PcP. All works fine and I am happy so far. My question is, rather than play local FLAC files from an external USB drive, is it possible to save them on the same micro SD as the OS and stiil be able to play them? If so, how would I copy them on to the card?

I'm very new to this, but loving it so far!

Thanks

Sean