Johnson, recently delisted by Melbourne after 69 AFL games, had no contact with the Hawks before his name was called out to join the side's training squad at Tuesday's NAB AFL Rookie Draft.
Despite his surprise at the unexpected lifeline he quickly agreed to join the club to battle Travis Tuck and Northern Territory winger Nathan Ahmat-Watkins for the final rookie spot, which will be decided after the NAB Cup Grand Final in March.
"Paul still wants to have the opportunity to play AFL football," Johnson's manager Adam Ramanauskas told afl.com.au. "He knows it's only a slight chance, but he just wants to give it his best shot and see how it goes.
"He's a mature guy, he understands the way AFL football works, but he still thinks he can play senior footy.
"He's keen to give it one last crack."
Originally picked up by West Coast with the 24th selection of the 2002 NAB AFL Draft, Johnson was traded to the Demons two years later for pick 29 which the Eagles used to secure onballer Matt Rosa.
Hawthorn endured a horror run of injuries to its big men in the lead-up to the 2010 season forcing Brent Renouf shoulder the bulk of the ruck burden with Simon Taylor (broken leg), Wayde Skipper (hamstring) and Max Bailey (knee reconstruction) unavailable.
The Hawks have added Johnson to their train-on squad, a new initiative by the AFL which only Hawthorn and Richmond chose to take up, in order to guard against a similar situation.
Coach Alastair Clarkson said his club had been up front with Johnson about the likelihood of him continuing his AFL career in brown and gold.
"In our situation with Johnson, we've spoken to him and said the chances of him actually being recruited with that last spot on our list are probably pretty slim," Clarkson told Melbourne radio station SEN.
"We took Paul because he can play both in the ruck as well as key position to give that support in case something untoward happens over the summer."
Tuck, who became the first AFL player to register a third strike under the League's illicit drugs policy in late August, is considered the front runner for the final spot with the club keen to aid him in his rehabilitation.
