Try this one.....First Create Table like this
DataTable dt_Exclusion = new DataTable();
DataColumn ExcCode = new DataColumn("fExcCode", typeof(string));
dt_Exclusion.Columns.Add(ExcCode);
DataColumn ExcDesc = new DataColumn("fExcDesc", typeof(string));
dt_Exclusion.Columns.Add(ExcDesc);
dt_Exclusion.AcceptChanges();
Session["ExclusionTable"] = dt_Exclusion;
Then Add a new row in that table
dt_Exclusion = (DataTable)Session["ExclusionTable"];
int Count = dt_Exclusion.Rows.Count;
DataRow dr = dt_Exclusion.NewRow();
dr["fExcCode"] = lnkbtnExcCode.Text;
dr["fExcDesc"] = lnkbtnExcDesc.Text;
dt_Exclusion.Rows.InsertAt(dr, Count);
dt_Exclusion.AcceptChanges();
Session["ExclusionTable"] = dt_Exclusion;
Then Bind Gridview
dt_Exclusion = (DataTable)Session["ExclusionTable"];
gvresultexclusion.DataSource = dt_Exclusion;
gvresultexclusion.DataBind();