Passing values from App part to Host web

Hello,

We have a list and we want to save data from external source into one field.

Example: On "add list item" page, we want to have some drop-downs. The data for these drop-downs should be loaded from external source. Due to some reasons, we cannot use BCS. So, we are planning to have some app part which will show these drop-downs with data. 

Is there any way to pass selected drop-down values to host web? We want to save these values in list item.

Thanks for your time!

S