Johnson, joined the club last week.
"He's an excellent goalkeeper and actually a player that I've tried to sign before," Johnson said of Marshall.
"His experience will be invaluable for us both on and off the pitch. It's vital that our senior players aren't just great characters, but also help us set the standard around the training ground and on matchdays.
"He is a fantastic addition to this squad."
The last of Marshall's 47 caps came at the delayed European Championship in 2021.
The former Celtic, Norwich, Cardiff, Hull, Wigan and Derby keeper started all three group games and was the hero of the play-off final penalty shoot-out in Serbia that ended Scotland's 23-year wait to play in a major tournament.
He has not featured for his country since after losing his place at Derby, where he had been captain, at the start of this season, but has been recalled to the international squad for a busy June, with Scotland facing up to five matches - starting with a World Cup play-off semi-final at home to Ukraine.
In January, Marshall moved to QPR and played 12 matches before picking up a hamstring injury in mid-March.
Matt Macey, 27, was Hibs' first-choice keeper this season, making 44 appearances, with back-up Kevin Dabrowksi, 23, taking the gloves on six occasions.
Liberia-born Kenneh has been capped by England Under-17s and Johnson commented: "He adds something extra to our squad and although his primary position is defensive midfield, he also offers us the versatility to play at the back.
"He's a young player with a big presence and a lot of quality. He has a real desire to develop and play first team football regularly. This signing is a real coup for us."
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