While anything is possible, it isn't likely and how would you authenticate such a rifle?
There were portable machine shops, and they probably did what repairs were practical but shortening a barrel to 98k is not typically a practical solution for a busy organization dealing with many more pressing repairs. I suspect if an upgraded Gew.98 came into a field repair shop (probably not often consider these were second line rifles and not front line rifles) it would be scraped for parts, or sent back to a depot.
Converting a Gew.98 to 98k is more involved than just shortening the barrel, and it would be more practical to re-barrel a Gew.98 receiver with a ordnance spare barrel, essentially using the receiver and some incidental parts mixed with ordnance spares and or salvaged parts.
If a field unit would go this far is anyone's guess, I suspect they had more pressing tasks at the front and most would just part out rifles that demanded significant work.
The main thing is how would you "today" determine a period field unit conversion or re-barrel from one cobbled up today, as while the Ordnance staffs and depots marked their work with acceptance, but I doubt a field unit would, and if we know anything about such field expedients (WWI it was common place, as Imperial-Weimar era they were rather frugal, but nazi era very wasteful and tended to use a lot of ordnance spares) they did not generally mark the rifles at lower level repair (Tikki Hut repair depots)
