Stand-off Holroyd, 36, will go straight into Tony Benson’s plans for Sunday’s league game at London Skolars.
He comes on a short-term deal to replace injured skipper Neil Roden, who will be out “for several weeks” with a severe dead leg.
Said club chairman Chris Hamilton: “Neil Roden is black and blue from his thigh right down his leg. As soon as we knew the severity of his injury we started to look for a replacement.
“We have done a deal with Graham Holroyd to see us through while Neil is out and then we will re-assess it.
“Graham watched us beat Barrow in the cup last Sunday and liked what he saw.
“He has already identified areas in which he can help us. In Neil’s absence, he is just the sort of vastly-experienced player we need in that position.
“We have a lot of young players in the side, particularly in the backs, and his know-how will be invaluable.”
Holroyd, a goalkicker, chalked up 650 points in 151 Super League games with Leeds, Halifax, Salford and Huddersfield between 1996 and 2003.
He has also played for Swinton and Doncaster, but he has been out of rugby league since the end of the 2010 season when he helped Halifax, in his second spell there, to overcome Featherstone 23-22 in the Championship Grand Final.
Acton, a relative newcomer to the sport, made an immediate impact at Hemel Stags and was soon selected for the Great Britain under-18s side to play the Australian schoolboys.
He turned in a man-of-the-match performance which aroused Wigan’s attention. He signed, moved up here with his girl friend and baby daughter, and played for much of last season and part of this in the Warriors’ under-20s Valvoline Cup side.
He also trains for mixed martial arts and cage fighting.
“I’m buzzing about starting this new chapter in my rugby career,” he said. “I was looking for a part-time contract and I had heard lots of good things about Oldham, so here I am.
“I haven’t played for a couple of months, but I can’t wait to get cracking in the reserves tonight.”
If Paul Ashton’s men are to pick up their third win in five outings they will have to avenge Sheffield’s 38-14 win, also at Whitebank, in early March.