Multiline text box in visitor group property

I was working on setting up visitor group with multiline property for string. Is there any option to include that? I've used following property but it gives only text box without multiline.

public string commaseperatedCountry{ get; set; }.

My requirement was to include comma separated country list from visitor group criteria property. If visitor country contains then display different blocks based on each country.

Any body have idea to implement this?