Hi Pradeep,
You can crate a Line type with the 'No Page Break' check box not selected, and attach that line type to your footer 'Table line' (Cell), this will do when your footer has not enough space in that page.
Create a table line of line type with 'No Page Break' check box selected and just put the same under the main area (line items), and in the conditions tab of the 'Cell' check the condition 'Only before the end of main window'. And in the cell create a text sy-uline(length). Length equivalent to your main area width.
Hope this will help You.
Thanks & Regards
Manu S