While you cannot create an Ossia directly in iPad version I have found that if you open a file with an Ossia from the desktop version you can copy and paste the signposts for starting and ending the Ossia and paste them into an iPad project and they will make an Ossia.
For those who don’t have the desktop version I have a attached a file with an Ossia that can be opened to copy the Ossia signposts ready to paste in Your own project.
Hope this works and is helpful.
Ossia for iPad.dorico (1.3 MB)