MadMango isn’t steering you wrong. Solidworks’ mirrored parts are ‘dumb’ solids, just like a STEP-in or Parasolid-in translation.
You cannot edit feature locations in any of the above, but should have no problem “filling them in” with an extrusion (have “Merge Result” on). That makes a seamless surface on which you can build the new features in the mirrored part.
Personally, I shy away from that because I like to control every single aspect of the mirror part with the Master Model (Parent part). This may seem complex, but after a few times it becomes second nature and does away with any editing of the dumb file.
If you’d like to try this, you can create an extra Configuration in the original Parent Part. This config contains the location and correct feature sizes that are unique to the Mirrored part. These unique features should be suppressed in all configurations
except the new config.
If you’re doing this prior to creating your mirrored part, you’ll get the configuration name for free when you Insert/Mirror Part. If you’re doing this after-the-fact (with parts you already have), you can create the configuration in the mirrored part and synchronize it with the “List External Refs” dialogue box. The “Use Named Configuration” dropdown list tells SW which config to look at in the Original partfile when the current config is activated in the mirrored part.
I hope that made sense. It’s my first post and all.
"I'm gonna go get the papers, get the papers."