Entella were relegated from the second-tier Serie B after the play-offs last season, but they have played just one Serie C game this season, due to an ongoing dispute.
The 36-year-old Cassano, without a club since Sampdoria terminated his contract in January 2017 and a brief spell with Verona, asked to train with the club based near his home in Genoa in northwestern Italy.
Entella said they had "accepted the request of Antonio Cassano that from Monday he will train with the first team... without any contractual tie".
The club are due to find out on Tuesday whether they will play in Serie B or C.
They played their opening Serie C game, a 3-1 win at Gozzano on September 17, before being given a lifeline to play in Serie B.
The Italian National Olympic Committee (CONI) decided a points deduction for Cesena should apply to 2017-18 and not 2018-19, meaning Entella would move up one place in last season's Serie B table and avoid relegation.
The club were told to stop playing in Serie C because they would be moved up a division.
But the Italian Football Federation (FIGC) said CONI did not have the authority for such a decision, with the case to be decided by a regional administrative court, on Tuesday.
Former international Cassano won 39 caps for Italy between 2003 and 2014. He won a Liga title with Real Madrid and Serie A with AC Milan.