Sync new music to iPhone without deleting existing?

Scenario:  I have Album ‘A’ currently in my Music on my i-Phone.  I have Album ‘B’ in my Music on my laptop.  I want to transfer Album ‘B’ to my phone, so I will use a USB cable and sync said Album ‘B’.  BUT, I want Album ‘A’ to remain on my phone, thus having both Albums ‘A’ and ‘B’ on said phone.  That’s what I can’t figure out, because when I sync album 'B', it replaces Album ‘A’.  The obvious work-around is to re-add Album ‘A’ to my laptop and sync it with Album ‘B’ to my phone at the same time, but there has to be a step I can take to avoid doing that. And that’s what I can’t figure out.  Any help…..Thanks.