@dzonatan in the latest version of Poser (there were versions which prevented negative scaling) you can set the X-scale of a mesh to -100% and you will have a mirror image. The next thing you need to is invert all the normals. This seems a strange thing to say, but Poser essentially ignores normals in OBJ files as it derives them from the facet winding order of the mesh. Using the Right Hand Rule, where the thumb points in the direction of the normal, the curled fingers follow the order of the facet vertices, so anticlockwise vertices will have a normal pointing towards you, and clockwise vertices will have a normal pointing away.
For a clothing item that defines a figure, you would need to import the mesh .OBJ into Poser without scaling. Since this comes in as a single prop, all the groups are preserved, and none of the vertices are duplicated, which is what you need. Select the imported object, set its xScale to -100% and choose the Grouping Tool. If the mesh doesn't have a group that contains all the facets, create one (it won't hurt when it goes back into another app, as long as you don't use the name of an actor for that catch-all group) and add all the other groups to it. Select Reverse Normals (since the xScale inversion will have reversed the winding order of the facets and made the normals point inwards), delete the catch-all group if you created one (or don't bother as it's not important) and spawn a prop. Select the new prop and export it as an OBJ file.
There may be some other caveats I've forgotten, but that should give you something you can use that's a mirror image of the original mesh. Oh, that reminds me. When you negate the xScale, the vertices will mirror about the mesh origin. Poser calculates an origin for a mesh when it imports it as the average of all vertices, so before the xScaling, you will want to set the object's origin to (0,0,0) which you can do from the properties tab (show origin then edit the coordinates) or Joint Editor.
The other thing that I've left out is making a mirrored CR2 to turn the new mesh into a figure. I'll leave the DS side to you, but for Poser, all the actors which aren't on the centreline need to have their xOffset, and xOrigin negated. Weight maps also need mirroring their xcoordinate, as well as morph targets.
I realise that I haven't answered your question in terms of Content Mirror for Poser, but if you haven't paid for it yet, you might find you can do without.
