If I understand your request correctly, you need to add a condition to modify only unfilled cells e.g.
Code:
Set oCell = oDoc.Tables(1).Rows(iRow).Cells(5).Range
oCell.End = oCell.End - 1
If Len(oCell) = 0 Then 'Add This line
oCell.FormattedText = oRng.FormattedText 'use this line to keep the formatting
'oCell.Text = oRng.Text 'use this line if the formatting is not required
Exit Do
End If 'And this one