Teenage midfielder Kolsi is viewed as one of the brightest talents in Finland and has been lured to Willem II from Jokerit by the Tilburg club's offer of a four-year deal.
"Kolsi is one of the bigger talents in Finland," a delighted Willem II technical manager Gerard Wielaert said.
"There was a lot of interest in him from European clubs, but he has chosen Willem II in part due to the positive experiences of his compatriots Hyypia, Kolkka and Koskinen here and because Willem II give youth players the chance to play."
Teenager van der Struijk has been rewarded for his efforts with a four-year contract, while Marcel Meeuwis and Frank van Kouwen have headed off to VVV-Venlo.
Meeuwis has signed for two years and van Kouwen for three, and Willem II have secured a buy-back option on the former in case he impresses over the course of the next two seasons.
