To do this, you need to use the ‘open in’ concept in iOS to transfer files between the one drive app and SmartOffice+.
Unfortunately I just tried this on my iPhone, and it seems the current OneDrive application does not support this feature for Office Documents. I think it may work if you open Safari on your iOS device, then go to the onedrive website and login and select the file from there.
To save files from SO+ to one drive, press the ‘File’ button, then ‘Share’ then find ‘Open in OneDrive’ in the scrollable list.
I hope that helps.