Backup external hard drive to the cloud via Time Capsule
Yes - but the flow of the data isn't linked directly.


The external drive can be opted in to back up to any Time Machine destination as long as there's enough space on the destination drive.
You can configure CrashPlan to back up files from the external drive - directly to the cloud.


Note: These are independent - you could enable 1 and not 2 or 2 and not 1 or both 1 and 2 as you wish. Getting Crash Plan to see files from within the protected Time Machine storage isn't something I would recommend.