Here's an approach using an ARRAY to hold a variable number of IN list elements. Instead of building an IN clause and using it within a WHERE clause, the UNNEST operator is used to generate a derived table from the array, which then is joined with the real table. (And here sa_rowgenerator() is used to mimic a real table.

begin
   -- use an array for the desired IN-list values
   declare s_id_arr array of int;
   set s_id_arr = array(3, 4, 5, 8, 10, 98);

   select list(foo_num order by foo_num)
   from
   -- dummy table via sa_rowgenerator()
     (select row_num as foo_num from sa_rowgenerator(1, 100)) SP
      inner join unnest(s_id_arr) as UN(ID) on SP.foo_num = UN.ID;
end;

returns 3,4,5,8,10,98

Now the next question is: How can you set the array contents outside this SQL block?

At least within DBISQL, it's not that difficult, say, via a stored procedure:

create or replace procedure SP_FilterViaArray(s_id_arr array of int)
result (list_of_ids long varchar)
begin
   select list(foo_num order by foo_num)
   from
   -- dummy table via sa_rowgenerator()
      (select row_num as foo_num from sa_rowgenerator(1, 100)) SP
         inner join unnest(s_id_arr) as UN(ID) on SP.foo_num = UN.ID;
end;

-- Use an ARRAY constructor as argument
call SP_FilterViaArray(array(9, 8, 7));
select * from SP_FilterViaArray(array(1, 2, 3));
select * from SP_FilterViaArray(array('3', '4', '5', '8', '100', 101));
-- or even build the argument dynamically via another sa_rowgenerator() call
call SP_FilterViaArray(array(select row_num from sa_rowgenerator(10, 21, 3)));

In addition to my ARRAY approach, here's a "less cunning" version with a simple comma-separated list of values as a string, using the builtin sa_split_list() procedure to turn that into a derived table. This should work with SQL Anywhere 10 and above:

create or replace procedure SP_FilterViaList(s_id_list long varchar)
result (list_of_ids long varchar)
begin
   select list(foo_num order by foo_num)
   from
   -- dummy table via sa_rowgenerator()
      (select row_num as foo_num from sa_rowgenerator(1, 100)) MyDummyTable
         inner join sa_split_list(s_id_list, ',') SP on MyDummyTable.foo_num = SP.row_value;
end;

-- Use a comma-separated list of values
call SP_FilterViaList('9, 8, 7');
select * from SP_FilterViaList('1, 2, 3');

How about creating a procedure with the arguments as comma separated long nvarchar and use that in your select statement:

create or replace procedure List2Table(
  in ValueList long nvarchar
)
begin
  select item 
  from 
    openstring(value ValueList) 
      with("item" nchar(40)) 
      option (delimited by ';' row delimited by ',')
    as "items"
end;

Then use this procedure in your query:

select
  list(foo_nm)
from
  my_table
where
  foo_id in (select item from List2Table(s_id_lst))

Another way to achieve this is by using execute immediate

begin
declare s_id_lst        char (30);
--
set s_id_lst = '3,4';
--
execute immediate 'select '||
                  'list(foo_nm) '||
                  'from '||
                  'my_table '||
                  'where '||
                  'foo_id in ('|| s_id_lst ||')';
end;

You could also create a long varchar variable for constructing the sql statement and then use that variable to execute the statement

execute immediate variable_Statement;

My method is a bit different but I use it a lot, often building lists from user queries/input. Works well and allows joins on multiple fields rather than just 1. The index speeds things up if the input list is big.

declare local temporary table "zt_tmp"( "myid" integer not null default-1, ) not transactional;

create index if not exists zt_tmp_id on zt_tmp(myid asc);

-- insert your values into the tmp table

select * from my_table as MT join "zt_tmp" as Z on MT.foo_id = Z.myid;