In you Add Idea Page, The parent Should be BrainstormSession Dataview, inside Dataview
Add A Datagrid to show the list of Ideas,
Enable Multi Selection in the Datagrid Properties.
Add an Action Button Say It Select an Idea and Call a microflow, You microflow will now have two paramaters
Add a loop and iterate over Idea list and Associate the BrainstormSession object and at last close the page.
I have chosen Datagird since it has a Mutliselect option by Default. If you want to have a List view, you will have to tweak your Flow. The main aim is to have the BrainstormSession object as your Select microflow Parameter.
Another Way is You can change the Association to Many – Many and use a Reference Set Selector and right click on the Reference set selector and select Generate Page. Mendix Will take care of setting up the associations.
I would prefer the first solution. Good Luck !!