There is no way to export list columns to turn them into site columns other than saving the list as a template and then importing the definition into Visual Studio from the template. But even doing that you will eventually need to delete the list
columns and recreate them as site columns. I'm not sure exporting them as a template and using Visual Studio to re-implement them saves you any time over deleting them and rebuilding them manually.
Paul Stork SharePoint Server
MVP Senior Solutions Architect: BlueChip Consulting Group
Blog: http://dontpapanic.com/blog
Twitter: Follow @pstork
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