Although I have not done exactly that, anything is possible. I guess my first question to you would be why do you want to do that? That would help you decide when to leave the Frances.
www.rome2rio.com will help you find transport options.
This map from
www.gronze. com shows you that you could walk off the Frances Perhaps on the Invierno and take a bus from Lalin or Silleda.You could even take the Via de la Plata from Astorga to Zamora, then walk the camino from Zamora to Verin, and finally take a bus to Porto from Verin - or just take the bus from Zamora.
But whether you would do any of these options or not most likely depends on WHY you don't want to finish the Frances.
Another option you might like to consider would be to walk the Frances to Santiago and then turn left and head for Porto following the blue arrows.
View attachment 69501