Right-back Purkiss, 28, joined the Bulls from Oxford in January before leaving the club at the end of the season.
Midfielder Featherstone, 23, also left Hereford following their relegation from the Football League before joining Purkiss on trial with the League One outfit (pictured).
"Ben is a steady and reliable performer who has played the majority of his games at right-back, but can also play in the centre of defence," Walsall boss Dean Smith told the club's website.
With reference to Featherstone, Smith added: "He's a player that I have admired for some time. He's a midfielder who always makes himself available and keeps the ball well."