You've almost got it right yourself:

EXECUTE IMMEDIATE 'ALTER TRIGGER TRIG_NAME DISABLE';

Just remember that EXECUTE IMMEDIATE requires the command to be a string.

This can be put into a stored procedure like:

CREATE OR REPLACE PROCEDURE ddl_command (p_cmd IN VARCHAR2) IS
BEGIN
EXECUTE IMMEDIATE p_cmd;
END;
/

The procedure owners priviledges will dictate which commands it can perform.

Cheers