Dutch-born Moroccan El Hassnaoui, whose move is subject to international clearance, began his career with De Graafschap before moving to Sparta last summer.
The 24-year-old’s transfer was announced yesterday just hours after the club revealed that the highly-rated Nicholson had pledged his long-term future to the Tynecastle club.
The Moroccan Under-23 cap becomes head coach Robbie Neilson’s third summer signing following the arrivals of midfielder Morgaro Gomis and striker James Keatings.
Neilson said: “Soufian’s a player we see talent in. We can see his desire. We spoke to him to get an idea of what he’s like and we spoke to some people in Holland who knew him and played against him, and all the reports were good.
“It’s another development opportunity for us. We want to get him in and progress him. The first conversation we had, we told him he’s coming in here to develop but also to help us develop the young boys.
“He was brought up in the Dutch system so he’s got a very good tactical knowledge of the game and he understands the game. He can definitely help our younger players to progress as well.
“He can play up front on his own, he can play with a partner or he can play behind the striker. He’s very versatile and he’s got a lot of talent. He’s a good age as well.
“He’s still got ambition to move on to a higher level, which is what we need, guys that are going to come in and develop themselves, develop the youngsters and develop the club.”
Hearts director of football Craig Levein, meanwhile, has hailed Nicholson’s new three-year deal as the club’s signing of the summer.
The talented 19-year-old yesterday ended any uncertainty over his future by putting pen to paper on an extended contract.
Former Scotland manager Levein envisages welcoming up to eight new arrivals at Tynecastle but reckons that none will be as important as Nicholson’s decision to renew his deal.
After making his debut as a substitute in last August’s loss at Inverness, the Scotland Under-19 international went on to make a total of 26 appearances last term.
Levein said: “We’ll make a lot of signings over the summer and I’m expecting seven or eight players to come in but I doubt if any of those will be as significant as Sam re-signing.
“He showed great maturity last season, for someone so young. Once he got his chance in the first team, what impressed me almost as much as his ability was his workrate and his knowledge of the position that he played in. He’s got a willingness to help his team-mates and that is very valuable. From his debut he really lit the place up.
“He’s very much one of our, and Scotland’s, brightest young talents. It confirms to the Hearts support and everybody out there that we’re serious about developing young players.
“Sam committing for another three years is excellent news for everybody.”