Is it possible to cut an existing tub to make it a walk in shower?

Yes but it is not easy. Fiberglass tubs are easy to cut but getting a professional finish look is hard. There are some aftermarket companies, tub refinishers, who do advertise this although I have never seen their work. It may be cheaper to cut the old tub out and use a shower base.

I converted my mom's tub to a walk in after she got her knees replaced. I ordered a kit online from a company called convertabath. It took a couple of hours and wasn't to hard.