The short answer is that this is not really feasible. That being said, it is possible to run the Time Capsule and older version of the AirPort Extreme on the same network.......but.....and this is a big BUT.......
1) The AirPort Extreme cannot wirelessly extend the signal from the Time Capsule. It can only do this if it connects to the Time Capsule using a permanent, wired Ethernet cable connection......
AND
2) You must use a Mac running an older operating system like Leopard (10.5.x), Snow Leopard (10.6.x) or a PC with AirPort Utility installed on the device to be able to configure the older AirPort Extreme.
All things considered, It would be far better to sell the old AirPort Extreme....it might bring $10 or $15.....and add a new....or even a used recent model of the AIrPort Express or AirPort Extreme.....which can be configured with the Mac that you have running Mavericks or Yosemite. The extending device can be configured to either wirelessly extend the signal from the Time Capsule or connect using an Ethernet cable connection.