Greenwood, who was an outstanding performer for local amateur side Siddal in the 13-man code before joining the professional ranks, has already spent a short spell with Halifax RUFC earlier in his career.
He made his debut in a 29-19 victory over Doncaster at Ovenden Park in 2003, coming on for the last 10 minutes at centre, having also played for the second team in a 25-3 defeat by Sedgley Park.
Greenwood, who is the younger brother of Fax winger Lee, became the 1,000th player to appear in Super League when he made his Fax debut at Widnes in September 2002.
However, he found himself surrounded by controversy when he was sacked by Fax in April 2003 after he was convicted of causing grievous bodily harm following an incident outside a town centre nightclub. Halifax subsequently re-signed the player 15 months later.
Greenwood has been something of a journeyman rugby league player with Hunslet Hawks his most recent club.
"Gareth has agreed to join us and we will welcome him with open arms," said Morgan, who welcomed his new-look squad back for pre-season training this week.
"We now have a core of rugby league players in our ranks with Oli Marns, James Endersby, Iain Gordon and now Gareth.
"All have a good rugby union pedigree but Gareth has already made it known that he would like some specialised coaching, perhaps on a one-to-one basis to help him get up to speed with the demands of the game."
Another familiar face returning to Ovenden Park is Craig Barker.
The former Leeds Tykes stand off spent two seasons with Halifax between 2002 and 2004, playing just eight games, scoring one try and kicking five penalties for a total of 20 points.
And Halifax have also secured the services of ex-Henley Hawks lock Nick Corrigan.
Manchester-born Corrigan contacted the club with regards to a possible move after leaving Henley and moving back north with his work.
"Having Craig back will be a good string to our bow," said Morgan. "He has been at Wheatley Hills and has developed greatly as a player.
"He is somebody we are looking forward to working with as is Nick Corrigan.
"Nick more or less fell into our laps.
"He made us aware of his availability and we acted accordingly.
"He has experience of playing at a higher level with Henley and that experience could prove important."