In my experience, the only ways to enter a value into a popup menu are to type the item in or to paste it in.
Entering a 10 word sentence into a single item should be possible. I was able to type a 105 character string into item 2 of the pop-ups shown below.
Note: the numerical items (which used internal spaces or periods to prevent Numbers from interpreting them as numbers) were typed in as they appear, For the long string in A3 (menu item 2), I typed in 30 characters, then placed the insertion point before the first x in the second group, and typed in more, repeating this each time the insertion point got close to the left edge of the Inspector entry box, and before it jumped to the next row. This may not have been necessary, bu I'll leave that investigation to you.
The entry box, and display of the item in the popup menu are both restricted to a single row, and the menu itself appears to have a width restriction, so you may be unable to see the entire sentence in the menu itself.
Perhaps if you dropped the fixation on using a pop-up menu and concentrated on the end result you are looking for we might be able to offer a more useful solution.This one seems a bit awkward at best.
Popup menus of OTHER problems that come up so I generally avoid them. Can you provide context for WHAT you are trying to do? With that information we may be able to suggest ideas that will work in Numbers. Like Barry said pop-ups end up being awkward.
Also Number is NOT MS Excel and while it has some features of excell it does not contain all.