Thanks to Richard deeming my question worthy of a hint, I was able to find out that defining and then merging the range works:
private int curDescriptionTopRow = 8;
. . .
private void AddDescription(String desc)
{
int curDescriptionBottomRow = curDescriptionTopRow + 3;
var range =
_xlSheet.Range[_xlSheet.Cells[curDescriptionTopRow, 1], _xlSheet.Cells[curDescriptionBottomRow, 1]];
range.Merge();
range.Font.Bold = true;
range.VerticalAlignment = Microsoft.Office.Interop.Excel.XlVAlign.xlVAlignCenter;
range.Value2 = desc;
}