Yes, you can get a cat flap fitted to your back door on request. There are various options relating to pet entrances and your installer will be able to advise on the best one for you.
Depending on the type of pet entrance you want, you may need to consider a different style of door, as not all designs will be able to accommodate an extra aperture. While glass panels can be cut in a variety of ways, including a cat flap in an all-glass design will not normally be possible.
If you choose to include a cat flap in your door, you should also consider security features that prevent unauthorised entry - a collar that can be worn by your pet that activates the lock is a simple way of helping to prevent unwanted animals getting into your home.
Speak to your installer about what options are available to you, and they will be happy to recommend the most appropriate solution.