Take a look at the map published by IGN -
Visualizador de cartografía de los Caminos de Santiago en Europa creado por el Instituto Geográfico Nacional y el Centro Nacional de Información Geográfica con la colaboración de la Federación de Asociaciones de Amigos del
Camino de Santiago.
nco.ign.es
If you want to do this, you could:
1. Take Invierno from Ponferrada. Leave the Invierno at A Rua and cut across country to A Gudina
2. Take Invierno from Ponferrada. Leave the Invierno at Monforte de Lemos and cut across to Ourense
3. Continue on Invierno as far as Lalin, where the Invierno joins the VDLP / Camino Sanabres
4. Forget about the Invierno. Leave the Frances at Astorga. Walk south on VDLP to Granja de Moreruela, then turn West and follow the VDLP/Camino Sanabres.
Option 3. is straightforward.
Option 4. is lonely (as far as Granja) and long.
Options 1. and 2. require some creative / independent thinking. I don't see any tracks on Wikilocs for these routes, although there are tracks for various walks in the area:
Find the best outdoor trails for mountain hiking treks, bike tour itineraries, MTB rides, and up to 80 activities. Discover the most beautiful places in the world, download GPS tracks and follow the top routes on a map. Record your own trail from the app, upload it and share it with the community.
www.wikiloc.com
The distance in each case is 45 to 55km so you would probably need to find somewhere to stay overnight on the way. For option 2 that might mean somewhere in Viana do Bolo. For option 3, there are places to stay in Nogueira de Ramuín.