fix: Modify 'load' behavior to append data to existing sheet

Details:
- Added logic to detect existing sheet and append data if present.
- There is no direct way to append on the sheet, so I am reading the sheet to get the last row number and then writing after that row number.

Resolves: #5472

openbb_terminal/helper_funcs.py

@@ -1602,15 +1602,28 @@ def export_data(
                     )
 
                 elif saved_path.exists():
+                    # Load the Excel file to get the existing data
+                    existing_df = pd.read_excel(saved_path, sheet_name=sheet_name)
+                    # Get the number of rows in the existing data
+                    start_row = (
+                        existing_df.shape[0] + 1
+                    )  # Add 1 to start writing after the last row
+
+                    # Append data to the existing sheet
                     with pd.ExcelWriter(
                         saved_path,
                         mode="a",
-                        if_sheet_exists="new",
+                        if_sheet_exists="overlay",
                         engine="openpyxl",
                     ) as writer:
                         df.to_excel(
-                            writer, sheet_name=sheet_name, index=True, header=True
+                            writer,
+                            sheet_name=sheet_name,
+                            startrow=start_row,
+                            index=True,
+                            header=False,
                         )
+
                 else:
                     with pd.ExcelWriter(
                         saved_path,