If the album files were in separate folders and the individual filenames are difficult to recreate without the library information this workaround may help:
- Rename the missing library files inside MC using a simple rule. For example, use just the track numbers. Ignore the possible warnings about missing files.
- Name the newly acquired files similarly: 01.xxx, 02.xxx, 03.xxx etc and use a folder rule that places them in the correct folder structure. If you are going to rip the new files with MC use a different library for ripping and keep the original library intact.
- After you have the new files in place use MC tools for applying the library information and renaming the files properly.
Remember to make a library backup first and build a working backup strategy for the media files so you don't need to do this ever again.
I have found external USB2 or Firewire HDs useful for media file backups. I keep two backup copies of my about 500 GB of data. Currently I have a pair of big drives near my main server PC and another pair in a safe place. I keep the nearby drives powered only when making backups. I make incremental backups almost daily and exchange the backup drives weekly or so. The latest backups are always on the nearby backup drives and the safe place has another a bit older copy of my data. Sometimes I have made mistakes and made a backup before noticing them, but I have been able to restore the files from the older copy.