I can see a problem with that - I would assume that in fact not all the cells actually contain '3+4' so it's going to be difficult to know what to replace.
Followng might help, but without more details of your sheet, I can't guarante it.
I think you'll probably need to use some sort of macro to work through the sheet. I don't think recording one will work as it will only record the action for the specific cell you're working on.
Is the sheet really all just a bunch of numbers as described? I ask this because you will need some way to plonk an = in front of the cell contents, but don't want to do that if there is text etc.
If it really is just numbers, you can paste the code below onto a button - it will stick the = in front of whatever is in the cell, then move down to the next one. But if the result is an error, the macro will crash - e.g. if one of the cells has just an = in it, the end result will be == which is incorrect syntax for excel.
Change the 10 to however many cells you want to replace at a time. It will only work downwards, but you can experiment with the offset numbers to change that:
For j = 1 To 10
Dim oldv, newv As Variant
ActiveCell.Select
oldv = ActiveCell.Value
newv = "=" + oldv
ActiveCell.Value = newv
ActiveCell.Offset(1, 0).Select
Next j
Suggest you work on a copy of your sheet! If you need help to make this work, post again.