If you want to select multiple things, you could for instance add a data grid with multiselect and a button to do whatever it is you want to do, or use a reference set selector.
What kind of data are you working with that you want the user to be able to select?
To my knowledge no, but I would very much like to get suprised by a nice widget that does make this possible.
The input reference selector does something like that, but probably not where you want it.
The only widget I know of is the multi select dropdown that uses checkboxes: https://appstore.home.mendix.com/link/app/2295/
A workaround could be to have 2 references: 1-1 for the dropdown and a 1-* for the actual data selection. Use an On Change microflow on the dropdown that sets the 1-1 object as a member of the 1-* selection and than clears the 1-1. So every time you select something in the dropdown, it get's added to the list. If you select something that's already on the list, remove it.
But this a bit dirty and doesn't give the best user experience I think.