Margetts, 25, scored four times in five appearances before returning to his parent club earlier this month.
Manager Dave Frecklington said: "Jonny was great for us in his first loan spell, four goals in five games is a great return, he's a natural goalscorer and a real fox in the box.
"He came to us, got his goals and we won a couple of games. When he went back to Boston we said to him that if things didn't work out, you know where we are and we're absolutely delighted that he's back with us for the rest of the season."
Margetts had a spell on loan at Gainsborough Trinity earlier in the season and was named Matlock's Player of the Month for February.
Morley, 18, arrives from Burton Albion as cover for the injured Craig King.
Frecklington added: "Nathan's been training with us for two or three weeks and has done well. He needed to find a club and has joined us on non contract terms."