You mean, if you add like a 1TB external USB hard drive onto 500GB Time Capsule, it becomes 1.5TB?
And perhaps in the future, you add 2TB to it, and it becomes 3.5TB?
When you attach an external USB hard drive, it will be visible under the SHARED Time Capsule. But it will be a separate volume.
I understand why you would want to do this, since you want to have Time Capsule keeping all your backups and never want to run out of space. But I think it's better if you use it as a backup not as an archive. You know, in case you have a hard drive or hardware failure, you get a replacement and it's running the way as you had it before the failure.
If you need to keep valuable files, run archive. Or manually copy the files you need to keep.
Never tried or paid attention to anyone about combining multiple USB drives, but with Time Capsule, I don't think you'd be able to do this modification. Partly because you can't modify Time Capsule drive from Disk utility. You erase, name, archive your Time Capsule under Airport Utility.
How do you run out of space on Time Capsule so fast?
Do you move or add/delete a lot of big video files or something?
Or use Virtual Machine?
Time Machine only backs up modified files, so most of the time with some music downloading and making documents, my backups are only few MBs at a time.
I did notice when I use VMWare Fusion, that's categorized as being "modified."
So Time Machine backs up every time I ran VMWare Fusion.
Wasn't practical, since I don't need Windows on my Mac to be backed up every time I use it, so I put it on the do not back up list.
Since then, my backups take not even five minutes I think.