Was the name of the old machine different from the new one? In other words, in System Preferences->Sharing what name did you give to each computer? If they were different, you could try changing the names to match temporarily. Thus would give you access to the old Time Machine backups that you want. Keep in mind that you will need to change the sharing name back when you are done lest you won't be able to access the new Time Machine backups any longer.
Another option is just to open finder and browse manually on the old Time Machine hard drive and manually copy over what you want from the old backup.