You would have to create a new Excel sheet with a macro that can convert the column. That part is pretty easy, it would be something like this:
Sub Macro1()
Columns("nameofcolumn").NumberFormat = "0.00"
End Sub
The number format is irrelevant. The macro existing is the point. Then you would have a VC task that can call this macro and run it.
Task - Office - Office Macro
Then point you path and Macro name to what you just created.
That part is all easy - what the macro needs to do though is accept a changeable variable that can point to wherever your existing data is. That is the part I don't know since I do not work with Macros too much.
If you can sort that it would work.
Your data would be in File1
your macro would be in File2
you would use VC to run the macro while passing it a parameter or variable that it would take as the path and column number of the data you want to convert. So you would use a macro you build that is run by VC to change data in a different file.
The alternative, like Thomas said would be to read that column of data so it is an output in VC then create a new column with your data in the format you want.
So that would be a Get Cell - Cell By Reference Range
Let's say you want the third column
After that runs you have the data. You may need to do more manipulation to format it correctly.
Then do a Set Cell Task