if you want to do it with a one-liner (or look at the code to do it in a program), get excelPerl from ExcelPerl 0.12 released and try something like

Update: Sorry, I misread you. The code for ExcelPerl would be (if your col is number 2):

excelPerl.pl -ane "$C->[2]->{FormulaLocal} .= '+99'" -f test.xls
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If you have got a headline in the first row, just skip it with the additional parameter -h=1

Code to convert excel to csv with formulas (doesn't match the question):

perl excelPerl.pl -begin "use Text::CSV_XS; $csv = Text::CSV_XS->new( 
+{ binary => 1 } )" -ane "$csv->combine( map { $_->{FormulaLocal} } @C
+); print $csv->string, $/" -f test.xls
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which produces something like

a,b,a+b
1,4,=A2+B2
2,5,=A3+B3
3,2,=A4+B4
4,1,=A5+B5
5,5,=A6+B6
[download]