Option to use AirPort Express with 1 TB harddrive for Dropbox equivalent
You can surely do this but please not that doing so will void your Apple warranty with the Airport Extreme. 

What you need to do is:


Pull the HDD out of the Time Capsule (take it apart)
Install the HDD in an enclosure or caddy
Connect the enclosure to your Mac
Use Disk Utility to create a 100GB partition
Reinstall the HDD in your Airport Extreme and you are good to go!


When you partition the drive anything stored on it should not be affected since it is only resizing the current partition.