A little bit of additional information, the original poster (eckaner) was
probably thinking of MS Word where you can split up a single cell.
You cannot do this in Excel as each cell has to be addressable.
So you have to insert an entire column as Paul indicated. But then
to get the appearance that was desired you have merge the B & C
column cells row by row for all but the cell that was to be split.
There is an icon for this that would allow you to select
many rows in two or more columns and do this all at once.
-- Merge Across -- customize, commands, format
The icon is without color, looks like a spreadsheet gridlines with
the horizontal lines heaver than the vertical lines.
I have a page on merging cells, but the above will work without
using macros.
http://www.mvps.org/dmcritchie/excel/merge.htm