Lettany has signed after being with the reserves last season and Foy joins from Hallmark Security League outfit Cammell Laird, having previously been with Prescot Cables.
Ball was at Evo-Stik Northern Premier League First Division South side Northwich Victoria last season and has been on Celtic boss Liam Watson`s radar for quite some time.
That makes it a round dozen summer signings for the Celts as manager Watson looks to build on last season's consolidation.