A few comments.......
The Seagate drive will need to be formatted in Mac OS Extended Journaled to work well with the USB port on the Time Capsule.
The USB port on the Time Capsule is underpowered, so you may....or may not....need to use a powered USB hub with the Seagate drive, even if the Seagate drive has its own power supply.
The USB port on the Time Capsule is an old and slow USB 2.0 design, so backups/copies are going to be slow, and will take about twice as long as backups to the Time Capsule hard drive.
Adding the Seagate drive to the Time Capsule will not increase the storage capacity of the Time Capsule hard drive. You will still have two separate hard drives, each with their own storage capacity.