You will have to develop an RPA workflow to download the excel file → Read the file into a datatable and return it to the App.
Once you have the datatable, try assigning the datatable column value to the dropdown control.
This should be triggered on the event Loaded so dropdown will be ready with populated values from the excel file.
You can refer this for how to assign datatable values to dropdown. I hope it’s still relevent.
Creating a dropdown from datatable - Help - UiPath Community Forum