inserting values into arrays

I unsubscribed pgsql-sql for some days, so I don't know whether this
problem is reported already or not.

I am trying to write a function for making group and user creation
easier. I would like to insert a new value after the existing ones in the
field "grolist" in the table "pg_group".

I tried a few solutions without success:

a) Defining a variable in a plpgsql function with type _int4. But
the command SELECT INTO refused to take the array up. Or, if I can do
this, how can I append a row after the others?

b) The other idea was to get the values step by step from the array,
making a varchar type variable with the values and the other neccessary
punctuation ("{", "'", ",", "}"), and then perform an update on the right
row of pg_group. But I realized that there is no varchar->_int4 converter
in Postgres yet, unfortunately.

The strange thing is that I can do almost everything by typing the queries
across the psql frontend, but from functions nothing (I think it is due to
there is no varchar->_int4 conversion) .

What should I do?

Thanks in advice,
Kovacs, Zoltan