In the current version of the app it is not possible to plan a mission on your desktop and then upload it on the device. You can only fly one of the pre-defined missions. However, we will add your request in our suggestion list in order to be considered for future versions.
Indeed, you can import a .kml file containing a polygon corresponding to the flight boundaries. However, you have to select one of the pre-defined grid types to execute. For more information about how to import a .kml/.kmz file you can take a look in the following article: https://support.pix4d.com/hc/en-us/articles/218051003.