The 23-year-old winger had been training with the Daggers and scored his first pre-season goal against Colchester on Saturday.
Hemmings began his career at Wolves before embarking on loan spells at Cheltenham, Torquay and Plymouth.
He signed for Walsall permanently in 2012 and went on to make 32 appearances for the club but struggled to hold down a regular starting spot, resulting in another loan move away to Burton in March.
Hemmings played a key role in helping the Brewers to the League Two play-off final last term before being released by Walsall on his return to the club.
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