The process is the same as creating a mirrored subassembly. Once you have created the assembly you would like to mirror, place that assembly into a new assembly file. This makes it a subassembly now. Just select a plane to mirror about and the box around the whole assembly file to collect the parts. Then click the blue arrow to go to the next step in the process. Select the assembly and click the button for mirror, which is at the bottom of the property manager.
As a side note if you wanted to select multiple parts to have a opposite hand side, the process one would have to follow is same: select the part and click the button, select the next part and click the button. To quicken the process you can select the first part and click the button, then hold Ctrl and simply start selecting the parts down the list.
Once you have created the mirrored assembly, dissolve the mirrored assembly. Then you can open that mirrored assembly in its own file and save it.
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