The Union will be the 16th franchise in the league beginning this season.
On Wednesday, the Union sent their No. 1 allocation ranking to D.C. United, allowing D.C. to pick up Troy Perkins, the 2006 MLS Goalkeeper of the Year.
In return, the Union got allocation money, D.C. United's seventh overall selection and Brazilian midfielder Fred.
The 6-foot-2 Mwanga scored 14 goals and had two assists in 18 games for Oregon State last season. The 18-year-old was the Pac-10 Player of the Year.
