You can try handling ElementExporting event and set the Value from the argument to the one you want. As for setting the ToolTip, it depends on how it is defined. If it is at column level, then you can find it easily and do the following:
public void OnExporting(object sender, GridViewElementExportingEventArgs e)
if (e.Element == ExportElement.Cell)
e.Value = (e.Context as GridViewDataColumn).ToolTip;
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