I forgot to mention in my cautions above that the USB port on the Time Capsule and other AirPorts is underpowered....(it was originally designed to support a printer).......so you often need to use a powered USB hub with an external drive connected to the USB port on the AirPort.......even if the external drive has its own power supply.
Time Machine backs up differently to a USB drive connected to a computer......(this is called a "local" drive)......than it does to "network" drive attached to an AirPort router.
So, the backups that you have made with the USB drive will not be useable if you try to connect the USB drive to the Time Capsule and back up that way. Time Machine will start all over again with a new complete backup of your Mac.
Not sure how much time you want to spend trying to do something that I do not recommend, but if you want to pursue this, the first step would be to connect the USB drive to the Time Capsule and then see if the drive appears when you open up AirPort Utility, click Edit and then click the Disks tab at the top of the window.
When you do this, you will see the Time Capsule drive......(named "Data", unless you have changed the name of the drive)......along with the name of the Seagate drive.
If the Seagate drive does not appear when you do this, then the next step would be to try using a powered USB hub to see if things will work that way.