Walsma debuted in the Hoofdklasse in 2005 and went 1-6 with a 5.11 ERA, walking 38 in 44 innings for the Amsterdam Pirates; he was 5th in the league in free passes. He fell to 1-6, 6.75 in 2006 with 40 walks in 371⁄3 innings, finishing 6th in walks. He participated in the MLB European Academy that summer. In 2007, Walsma was 1-2 with a save and a 7.62 ERA for Amsterdam, with 19 hits and 13 walks in 13 innings.
In 2008, he began the season 1-4 with 5 saves and a 4.22 ERA, fanning 48 in 422⁄3 IP. Walsma was added to the Dutch national team for the 2008 Haarlem Baseball Week. He allowed one run in 62⁄3 IP in the event. He then was selected by coach Robert Eenhoorn in the team that represents the Netherlands at the 2008 Summer Olympics in Beijing.