Yeah, you can continue to put progress tokens on the quest even if there are already 8.  I believe a FAQ entry noted that there is no upper limit on how many progress tokens may be placed on a quest card.

This is nice because there are some goblins that will remove tokens from the quest when they attack.

peke12 said:

You still add progress tokens on the quest card, even if it already has 8 tokens.

Hey fellow i just got a Nightmare pack and try to play JDTA solo with Thorin company deck……… Crazy challenge i dont believe is possible solo at all.

There is some nighmare  cards too powerfull just dull. Anyway you can make whole SOM cycle exiting again to add some cards there and there……

This is possible. I think it is in the FAQ, but you can put any number of tokens on a quest, even beyond the printed number on the quest card. And it is advisable to do so while you are killing the hill troll. We got 8 on the quest on our 1st turn, and did not put any more on (we didn't know you could), and then as my son slowly battled the hill troll (in our 3 player game), we watched half of those 8 progress tokens disappear from encounter cards and shadow effects.  It all went downhill from there. We still have not beaten Journey Down Anduin (2 player…wife and me, or 3 player…wife, son and me).