Is there a way to "mass add" text into a column of information?

Example: We do tractor parts and all info is kept in Excel databases. I
need to update part numbers. Right now, the column looks like this:

11111
22222
33333
etc.

and we need it to look like this:

D11111
D22222
D33333
etc.

Is there a way to tell Excel "add a letter or number at the beginning of
each cell in this column"? Same thing when adding category codes for our
website. I have everything broken down by model number, and have to manually
edit our codes. It would be soooo much easier to tell Excel to add the code
information before and after whatever is in each cell. Any way to make this
happen?

Thanks for your feedback.