Put your data requiring a negative into columm "B"
Into another column you've created (lets say column "A") place a minus
sign.

Place this into column C: =A1&B1

Copy down all of column C, and you've got negative numbers.
If you need to replace column B, copy, and paste as values all the data
in column C, to wherever you like, back to column B, you may delete
column A.

Pierre