No, the linear concat already needs to have been created for the first array set to have the proper metadata, from that point it's possible to grow the array just by adding arbitrary sized mirrors to the concat array, then growing the file system.
However, it is possible to convert an existing JHFS+ volume into a Core Storage Logical Volume without destroying the data on it, and also without enabling encryption via a Logical Volume Family. Next, you can create the 3TB mirror array, making it a Physical Volume, adding it to the same Volume Group as the first mirror, and then growing the underlying Logical Volume which will also grow the file system. I've also only done this with diskutil cs commands on command line, the Disk Utility GUI application doesn't have much of Core Storage exposed at all. The catch is, in either case, if two drives go in a mirror, it won't just take out that mirror, it takes out the entire file system. So you still must make regular backups.