The zero fret would have to be planned for in the guitar design. You couldn't add one later without major adaptation of the neck and fingerboard.

I have one guitar (an Epiphone) with a zero fret. It plays and sounds as nice as any of the others. The idea with it is to have all notes sound the same, i.e. all are fretted, and it reduces the job of the nut to lateral string spacing.

Many luthiers say the nut can function as both string spacer and "first fret" so why bother with a zero fret? Also they like to be able to work on the nut with their little files for set-ups and finetuning too.

Six of one and half-dozen of the other. A zero fret would neither persuade or disuade me from buying and playing a nice sounding and feeling guitar.