You cannot use a button to add new fields to a table. But you can add new records to a related table. Define a related table of keywords with one record for each such word. Then add additional records whenever you need to add more keywords.
And you may also want to set up a join table to implement a many to many relationship between a table of keywords and the table you want to search. That way, a single keyword can be linked to many records in your main table but a given record in you main table can also be linked to many key words.