This is one of my biggest frustrations with aviation— the certification required to get this done is hugely onerous. The whole basis of certified aircraft is that they may not change, which makes improvements like airframe parachutes, auto land systems, and even terrain awareness, engine monitoring, etc. very costly to obtain. I think there is an argument to be made that there should be a pathway to airframe recertification to allow for innovation and improvement to take place in the aviation industry.
Instead, the FAA is probably going backwards on this issue and doubling down on the regulatory framework that gave us the MAX-8 situation while narrowing any avenue for smaller firms to innovate [0]
There is simply no way to retrofit a parachute into an existing airframe. The airframe has to be designed around it from the start with appropriate stress points.
There are retrofit ballistic recovery systems available as a Supplemental Type Certificate (STC) for several existing airframes, e.g. https://brsaerospace.com/cessna/