The .musicxml extension requires MusicXML v.3.1 or the newly released v.4.0. Since neither versions have seen wide adoption, don’t expect this in the next Encore release. Even Sibelius skipped v.3.1 and just got 4.0 only a few weeks ago when MakeMusic (Finale) released the Dolet 8 Plug-in for Sibelius (won’t work with any other app).

Overture supports MusicXML version 3.0, as does the current versions of MuseScore, Dorico, Sibelius (without the plug-in), Finale (3.0 is in the drop-down menu) and Notion so this is what I expect.

MusicXm is a toolkit, not a feature that one just adds. How well it is implemented and what features it includes depends on the developer. Some, including Overture, do this better than others.