Ian Holloway's side sit just two points above the Championship drop zone, having played a game more than 22nd-place Yeovil Town and four more than basement club Charlton Athletic, as they enter their final 11 games of the campaign.
DJ Campbell, Jermaine Easter and Steve Morison are all injury doubts for Saturday's derby with Charlton Athletic and the 31-year-old could make his debut in the clash.
The former Austria international lists Bayern Munich, Rapid Vienna and Cologne among his former clubs, while he has also enjoyed spells in England before.
He joined Wolves in 2009 and was loaned to Bristol City a year later, with Maierhofer last playing a competitive fixture in May 2013.
That came with Cologne, for whom he scored once in 14 appearances in Germany's second tier.
"The 31-year-old trained with his new team-mates on Thursday and (Friday) morning, and could go straight into the Millwall squad for Saturday's game," read a statement on the club's website.
Holloway will be eager for Maierhofer to hit the ground running, with Millwall having taken just three points from their last possible 12 - scoring only once in that time.
