Is this possible?

Well I am thinking of making a Necro on my account and I had a question. If I get a skill on say my W/N which is a necro skill but it's say from prophecies and my necro is in nightfall. Would it be able to somehow use/get the skill I capped our bought from my warrior?

Yes. If the skill is a core skill, you can buy it from any NF skill trainer. If not, then you will need to visit a Proph skill trainer to get it.

Alternately, you can just use a necro skill tome.

Any skill that is "unlocked" on your account - either by a character having it or you used Balthazar Faction -- can be obtained by any of your characters by use of a skill tome.

I quite often use my main character to unlock skills via purchase or capture for the use of my heroes and other characters. She has access to all areas of the campaign and can visit any Skill Trainer. Thus, my necro got to use Spiteful Spirit very early in her career.

I see, well how much would a necro tome cost (elite and normal)? I have some money to spare but I wouldn't like to spend like loads.

normal tomes cost less than skills from skill trainers, like 400-500 or something
elites cost 5-10k depending on the class.

Yeah I so the same, kinda sad to buy 1k skills my char will never use and a tome but it makes those early days very easy, that's why I wish every skill was a hero skill I love those.