And he will be joined at the club on Sunday by Wigan's Gary Connolly.
Endacott has been working with Stuart Spruce at the Halton Stadium and helped them avoid relegation from Super League following the departure of Neil Kelly.
St Helens, meanwhile, have made Michael Smith, the Castleford Tigers and New Zealand Test forward, their third major signing for next season.
Smith, 26, previously played for Canterbury Bulldogs and Hull before joining Cas, who will be replaced by Leigh in Super League next season after being relegated.
Saints boss Ian Millward said: "Michael will add size and mobility to our pack and we aim to make him an even better player at our club."
Smith, who has agreed a two-year contract, follows Bradford prop Paul Anderson and Aussie Test centre Jamie Lyon to Knowsley Road. Tweet

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