Tonal pedal addition

Certainly. It is done regularly, especially in refurbished pianos coming from Japanese suppliers, such as for example the G series which were born with two pedals.

It is not a difficult job, but it becomes long if the damper levers do not have the tabs for the tonal bar: in this case, they all need to be replaced.

Logically, I deduce that one must also mount a new lyre with three holes...

If there is space, it can be modified there (and often there is space, see Steinway, see Yamaha for example).