As Mogens mentions, MusicXML handles sounding microtones by using decimal values in the alter element. The alter element uses semitones, so 0.5 would be 1/4 sharp and 1.5 would be 3/4 sharp. MusicXML also has many microtonal accidentals in the accidental element, including the quarter-sharp and three-quarters-sharp accidentals used in your example.
Your file exported from OpenMusic contains neither microtonal alter elements nor microtonal accidental elements, so there is nothing that any importing program can do with this in terms of microtonal music. The version of OpenMusic that you are using has many other MusicXML export problems. You can see this discussion on the OpenMusic forum about some of the progress being made to improve OpenMusic's MusicXML export: http://forumnet.ircam.fr/user-groups/op ... post-18551
Please contact the OpenMusic developers about your request for better microtonal export to MusicXML files.