Yes, the REX adapter supports this operation. See "Add Organization Technical Administrator" as an example. You'll want to note that the REX adapter ships with the IAC product, not Cloud Portal natively.
"Add Organization Technical Administrator" service is using REX Add Person agents which will update "Organizations" attribute for one user, but what I need is to populate "Organizations" attribute of multiple users with multiple OrgUnits or the other way, adding multiple users as a member of one or multiple OrgUnits.
Can I use "Cloud Portal Update Service Request" or " Update Multiple Service Request Parameters" processes in Tidal for this? I believe I can send Users and OrgUnits list from Portal to Tidal, then use Tidal to call back Portal and update "Organizations" attribute for each users with multiple values (Orgunits).