You can add full text to an existing instance by changing the SQL Server program in Programs and Features. Follow the steps below. You might need the original disk or ISO for the installation to complete. (Per HotN's comment: If you have SQL Server Express, make sure it is SQL Server Express With Advanced Services.)
Directions:
On the Installation Type screen, select the appropriate SQL Server instance.
Advance through the rest of the wizard.
Source (with screenshots): http://www.techrepublic.com/blog/networking/adding-sql-full-text-search-to-an-existing-sql-server/5546