Your question is a little unclear...as the part that you indicate you want to bold in Excel is a DataGridView in the import from word method. Do you maybe want to bold the first row in the excel document?
using xl = Microsoft.Office.Interop.Excel;
xl.Range rng = (xl.Range)xlWorkSheet.Rows[0];
rng.Font.Bold = true;
Simple as that!
HTH, Z