You can use the same drive, and keep your TM backup of the MBP. TM will start a new one. They can both reside together on the same drive. You'll be able to browse the MBP anytime you need to, but the iMac will have it's own, separate backup. (That's all assuming you have enough free space for both.)
So, yes, you just plug it in and start using it for your iMac. The first time you plug the drive in, the iMac should ask if you want to use it for backups.