A mirror is a mirror geometrically speaking. However if you are concerned with assembly placement of the mirrored object you can capture this by using this method. You need Advanced Assembly to do this.
1. Create a new part (this will become your mirrored part)
2. Use copy geometry feature to copy surfaces from original side into your new model (use "solid surfaces" selection to grab all solid data)
3. Mirror the copy geometry quilt around your mirror plane of choice
4. Solidify the mirrored quilt.
5. Hide the copy geometry feature.
This is a work around I have been using the last few years.