Thanks John, for your inputs. A team mate of mine came up with an idea to load blanks.
We have to replace all null values(,,) with one blank space(, ,) and then when using the import wizard , you have to uncheck the check-box which says "Ignore leading and trailing white space(s) in file".
This would load a blank, which is not the same as null, but it works for our scenario or of course we can replace it with null in a calculated column if needed.
Thanks,
Benedict