Migrate old Mac hard drive to partition on new Mac HD, or external hard drive

Can I migrate old hard drive to an external hard drive (to boot from) or put old hard drive on another partition of my current Mac's hard drive? I need to retrieve encrypted files. My old Mac is dead. I don't want the migrated data to take over my new Mac because the old hard drive has activity from 3+ years ago. If the migration assistant isn't the solution, does anyone have any better ideas?