To answer your question directly, you will not be able to do that with the Notion for iOS app. To offer you a workaround, you can add a time signature such as 8/4, stack your whole notes, and then enter 8 more notes into the same bar. You must have a time signature in your score though. Unfortunately, there is not a way to hide a time signature on the page.
In Notion 6 for desktop, it is possible to hide a time signature. With Notion 6, you could add a large time signature, enter your notation, and hide the time signature. If you are interested in taking a look at Notion 6, you can download the demo from within your my.presonus.com account.