It is very simple to add a blank line to your SAP report selction screen via the use of the 'selection-screen SKIP' ABAP command.
*ABAP code SELECTION-SCREEN BEGIN OF BLOCK skip WITH FRAME TITLE text-s01. parameters: p_tinput like p2001-awart. selection-screen SKIP. parameters: p_rinput like p2001-umsch. SELECTION-SCREEN END OF BLOCK skip.