The striker left St James Park in 2015 before joining Notts County but spent last season with SkyBet League Two club Morecambe.
He now becomes Armstrong's latest eye-catching addition by joining Darlington and the former Blyth Spartans boss believes Campbell has the ability and motivation to return to the professional ranks in the near future.
He told the Evening Chronicle: “Adam is massively motivated to get back into the professional game and I think we can help him do that.
“I have asked him about why he has dropped to our level so quickly.
“He believes it is just because others have seen him as a winger and played him out of position on a regular basis.
“Everyone raved about him as a kid and rightly so because he was electric.
“I think he has been over-coached as a youngster and that has maybe taken something away from him but my players are always allowed to express themselves and I think we can help him regain that spark he had as a kid.
“He has been signed as a No 9, that is the only position he will play, and we can get him back scoring goals.
“He will catch the eye if he does that and he can reach those levels once again.
“We can get him there - it will not happen overnight but he can do it."