Yes, you can start ExoCross in Windowed mode.

1. Open Steam
2. Click on ExoCross in your Steam Libary (but don't start the game!)
3. Click the Gear icon on the right side of the Steam window
4. Click Properties
5. Add the following text to the 'Launch Options' text box.

-windowed -w 1920 -h 1080

Note: You'll need to adjust the numbers in the text to change the window size.