Pavetim,
I just went through this, and unfortunately, getting your retirement date (once set) extended for any type of medical care is difficult, if not impossible. You can work with your PEBLO to send in a memo requesting an extension, but I was informed nearly all requests are denied. I sent in a request to have my retirement date extended just 30 days so I could recover from a surgery I had for a service connected condition and they still refused. You should look over the reg and see if there is a path forward for your situation. I am Air Force and tried to get an extension under the loosely defined "hardship" clause.
Best of luck to you, and should you have any success getting your extension approved, please post back on here so that others can be encouraged that such a feat is possible.