I am not sure how to do it exactly. It is done on the bind process. If you are using the SPB, there is a bind option in the environment options. If not, you would have to manual bind the SP with the appropriate isolation level.
There is a way to set the isolation level for each select statement; this overrides any isolation level current in effect. just add the WITH <is>
where <is> is either RR, RS, CS, or UR. I have done this and it works fine.