Executing Dynamic DDL

i am trying to execute an 'alter table' statement dynamically.. it
seems that "execute" only works with DML..

is there any way to execute DDL statements??

here is what i am trying to do:

--------------------
create or replace function em.process_table (
    p_table              varchar)
returns void as $$
declare
    v_check      bool;
begin
    -- Add Creation TimeStamp column if it is not there.
    select count (*)
    into   v_check
    from   em.all_table_columns
    where  tablename = p_table
    and    columnname = 'creation_timestamp';

    if v_check then
        execute 'alter table em.' || p_table || ' add creation_timestamp
timestamp not null';
    end if;

    return;
end;$$ language plpgsql;