I don't think the symmetry modifier works that way, seeing as how it has a "Weld Seam" checkbox.
If you have an iteration of you model that is symetrical you can use my Mirror Morph target script. You need the symetrical model to match vertices, then you can apply the mirror.
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If you have an iteration of you model that is symetrical you can use my Mirror Morph target script. You need the symetrical model to match vertices, then you can apply the mirror.
http://www.juantwo.com/2006/04/mirror-morphs-on-x.html