Hi Daichi,
you can try any of these widgets
Dropdown ReferenceSelector extension showing multiple attributes
https://marketplace.mendix.com/link/component/112261
Customizable dropdown
https://marketplace.mendix.com/link/component/203058
Hope it helps!!!
hi Daichi Miyuki,
1.You can use microflow to add all the roles to one attribute by the Loop(Note: No commit, on new dummy Role object while create and adding to the Attribute) and Return the microflow to the Drop down.
2.Or you can use the mendix market place component https://marketplace.mendix.com/link/component/117326