Time machine drive used for time machine before..can it back up a new drive without erasing?
Yes, you should have no problem doing that.  Simply tell Time Machine to backup to that drive, and Time Machine with create a new folder in the backups.db folder on your external drive.  As far as Time Machine is concerned, your new backup will be as if it is from a different computer, even if you're using the same MacBook Pro with a new hard drive.  Time Machine can handle backups from multiple computers to the same drive.