Not unless someone writes a an app that will let you browse inside a zipped archive, edit a selected file, and rezip back up while keeping the ePub's structure intact. Perhaps Calibre's explode command will do this, though I'm not sufficiently familiar with using it to say for sure.
I don't use Sigil myself, but perhaps it's possible to somehow manually edit the OPF it produces before generating the final zipped ePub?
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