You cannot. You can insert an entire column or an entire row.
Other options to make it look that way
--Enter some data in cells A1:A3 and in cell B2. Select A1:B1 >Right
click>Format cells>Alignment>In 'Text Alignment' under Horizontal select
'Centre across selection' and click OK. Do the same with A3:B3
--Another option is to merge the cells (not recommended due to various
reasons)
Select A1:B1>Right click>Format cells>Alignment>Under 'Text control' select
'Merge' and hit OK.