As far as we know, BIMX format is only used in Graphisoft's ArchiCAD BIM software, so there are really not many viable alternatives how to achieve dwg to bimx conversion - if any. It doesn't look like any 3rd party software supports BIMX format, so using the original software is your only option.
It's not even possible to recover or export data from .bimx files back, it's just format exclusive to ArchiCAD.