Hi Margautz! Welcome to the forum!
From Pamplona to Ponferrada, train is your best bet. Take a look at the web of RENFE (
www.renfe.com) for info and ticket purchase. As there's just one direct train (and it makes a long route), I suggest to book it in advance.
Alsa has seasonal direct bus service from Pamplona to Ponferrada twice a week but it ends in August so it's not an option for you.
If you suggest to go further after Pamplona, then I can walk other days (until where?) and take the train/bus to Sarria or Lugo (or anywhere you can suggest) to end the Camino always in Santiago.
You could walk till Logroño. Once in Logroño, you could take a bus to Lugo with Alsa (
www.alsa.es/en) or the overnight train to either Lugo or Sarria (take a look at the web of RENFE for info about the overnight train).
Estellesa bus from Pamplona to Logrono, Autobuses Jiminez to Burgos, and ALSA from Burgos to Ponferrada.
If traveling on Friday or Sunday, there's an easier bus route: Bus from Pamplona to Vitoria with Autobuses La Unión (
www.autobuseslaunion.com) and bus from Vitoria to Ponferrada with Alsa.
You might consider taking Renfe train from Burgos to Palencia and Palencia to Ponferrada.
There are direct trains from Burgos to Ponferrada. No need to change in Palencia unless the direct trains are full.