Since those are not your tunes I bet it shouldn't happen automatically.
If it was built in I'm sure the owners of the music would squawk loudly as what stops us from merging with any other user?
I have an iMac. My wife is one user of that iMac. I'm another. We each maintain different user accounts on that one iMac so that we can keep our email, contacts, appts, etc. separate. We also each have our own separate iTunes accounts with which we've each bought songs, etc. using that one iMac. In other words, all materials bought through iTunes are on that one hard drive (albeit under different user accounts).
How can I get my iTunes account (on that iMac) to include her iTunes purchases (on that same iMac)?
Appreciate any help in advance.