old iMac TM HD to a new iMac
Option-click the Time Machine icon in your menu bar with your Time Machine disk connected, then choose Browse Other Backup Disks and select your TM disk from the list.
This lets you browse the backed up data on the TM disk from your new machine using Time Machine.app and choose individual files or folders to restore to the new Mac.

If you choose to add the Time Machine disk as a backup disk to your Mac, this will create a separate backup history for your new Mac alongside the old backups, not merge the two. You can manually associate the old backups with your new Mac to use a single history (How to reassociate Time Machine backup with the computer), but I wouldn't recommend this unless you restore the entire backup to your new Mac first.