You won't be able to type an accented character directly into Arbor when adding someone's name - you'll need to copy it from another source, and paste it in.
You could search for the character on Google and copy it from there.
Alternatively, go to a word writing document, such as Microsoft Word or Google Docs. Find how to enter a special character. For example, in Google Docs go to Insert > Special Character.
Find and click on the character so it appears in your document.
Copy this letter, ready to paste later.
Go to the Identity section of the profile in Arbor and click on their name.
Click the orange Edit button.
Remove the letter you wish to replace with the accented letter, and paste in the new letter in its place.
This is what the profile will look like after this has been added.
Preferred names in searches
If their preferred name also includes the accented letter, it will not be recognised when searching for the name unless you use the accented letter in the search too.