Does anyone out there know how to auto-insert a zero into a blank cell?

I have a spreadsheet that I need to import into an Access db. It's not a
huge spreadsheet, but it's not small either. I have 600 rows and 10 columns.

Example: this is what it looks like...

date salesperson cat1 cat2 cat3 cat4 cat5
etc...
1/1/01 Jason 10 5 1
6
1/1/01 Jason2 5 7 6 4
9
1/2/01 Jason 2 4
4 8
And so
on................................................ ..............................................

I need to convert all of the blank cells into zero's.
This must be done either before the import or in Access after the fact
(whichever is easier.)

I just can't seem to think of a way to do this... Your help would be greatly
appreciated.