Cowperthwaite, 27, won the National League North with Barrow and has also been with Morecambe, before having a prolonged absence from the game with an ACL injury.
Former Luton, York, Barrow and Chorley striker Jason Walker arrives following Workington’s relegation from the EVO-STIK League Premier Division.
Boss Mark Fell said: “We’re a good fit for Jason, we created a lot of chances last season and the more we can compliment our forward line to convert those chances the better.
“We’ve a fantastic stable of strikers who all offer something a little bit different and Jason will fit well into the group.
“I spoke to him at length and was impressed with his outlook on the game, he’s clearly still got a real desire to succeed that’s important, he’s a naturally fit lad who looks after himself so he’ll hopefully hit the ground running in pre-season.
“Niall is a class act, he’s proven to be able to play successfully at a higher level, he’s fit and ready for a new challenge.
“We’ve kept tabs on him for a while now, there was nearly a deal done a few months ago to get him last season but it didn’t work out, so we’ve had to be patient but now is the right time for him to come into this squad and he’ll make us stronger.
“There’s the added benefit of his versatility, I’ve seen him play at right back and left back and he can fill in at centre half, but ultimately we want him to be working in the engine room of midfield.”