I've asked this question before and for the last year I've been able to dissolve a file until now. 

*In System Options>Import, enable 3D Interconnect.

*Then drop down the File Format list and select STEP/IGES/ACIS. ​​​​​​​Here you can choose how you want the imported file to be structured.

*With 3D Interconnect on, after import, right click the root feature in the tree and select Dissolve Feature (then Yes,