No, you have to actually own the corresponding campaign to be eligible to purchase skills from that set. For instance, if you buy the Nightfall unlock pack (such as you have), you have to own the Nightfall game to take advantage of it. I believe it actually states that on the in-game store too, several times.
Regardless, you still can't buy a skill that requires a Sunspear rank. You need to actually do the quest involved and/or earn the required rank to gain/get the skill.
If you do a search on the wiki for sunspear rank, you will see the following:
'After reaching the rank of Sunspear Sergeant, the following profession-specific Sunspear skills can be purchased for 1 hero skill point from Pikin, Shiloh and Libeh, the three Sunspear hero skill trainers

it then provides a list of skills)'
Hope that helps.