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When importing emn files directly in Creo Parametric, is it possible to set the default import of emn files to Assembly mode instead of Part mode? According to PTC eSupport article - CS398256, It's not part of Creo Parametric functionality.
Now I recommend that PTC improve in future Creo releases.
You should be able to create a mapkey that will select assembly when importing .emn files. This will provide a workaround while you wait for this enhancement to be adopted.