Ideally you would use the alter table statement:
alter table g1 options (set cardinality 100)
But that is currently only for load time DDL - an issue could be logged to make it applicable to runtime as well.
Alternatively there is the system procedure setTableStats:
call SYSADMIN.setTableStats('fqn', 100)
Thank you Steven for the reply but I guess this will not work for foreign temporary tables?
Yes Bram you are correct, the procedure only works for non-temporary tables. I should have remembered that you can add the cardinality as an option in the create:
create foreign temporary table x ... options (cardinality 1000, updatable true) ...