your problem is actually not a problem and its a very common thing in home improvement projects. All you need to do is a extension jambs, get some flat board and cut stripes in thickness (width) of level difference and nail them to visible on your pics 2x4's on sides and top of the opening. Then cover the gap between installed piece and sheetrock with casing, regular casing job like around doors, windows etc. Caulk, paint and enjoy !
Please refer to the attached picture.