need to add blank value to text attributes

Hi Experts,

I have a requirement to add blank value to attribute list. the requirement is mainly for a product linked to one category and the corresponding attributes are having values. Now I would like to change my product category from existing one to another, so for this first I need to remove texisting values of attributes linked to that category. if I don't remove my attribute values then the existing attributes will link to new category automatically when I change my product category.so can you tell me how to add blank values to text attributes so that i can select blank value for the existing attribute.

Thanks,

RDNPrasad