El Hassnaoui, a Dutch-born Moroccan 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 internationalist is head coach Robbie Neilson's third summer signing following the arrivals of midfielder Morgaro Gomis and striker James Keatings.
"Soufian's a player we see talent in," Neilson said. "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. 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."