From the Studio One 3 Reference Manual found inside Studio One 3
Importing Project Files from Other Applications
Studio One can open several other application project file types. These include PreSonus Capture™ Sessions (.capture), Steinberg Cubase Track Archives (.xml), Steinberg Sequel Projects (.steinberg-project), Kristal Audio Engine Projects (.kristal), and Open TL (.tl). To open any of these project file types in Studio One, navigate to File/Open, and select the desired file.