[X4U] how to sync one folder to another?

[Ben Gold]

I have 2 mp3 folders with some overlap of Artists and such, call them 
Folder A & Folder B.

Folder A is much smaller than Folder B.

I want to "synchronize" Folder A into Folder B.  Every sync utility I 
look at seems to want to synchronize the two folders so that they're 
the same (which is what synchronize means, I know).

What I want is  a sort-of one-way synchronization.

If file foo/bar/goo exists in folder A but not in folder B then copy it 
to folder B.  If it does exist then don't.  But it has to recurse down 
all the levels of folders, "synchronizing" folder B so that it contains 
everything in Folder A.

It looks like rsync might do it, but I'm worried about using it because 
psync was written because rsync doesn't transfer all the resource 
files.  Is that correct?

any info appreciated,

Ben