One solution is to add a hidden element to the document that contains the alternate spelling.
Another solution is to use the <oneof> element. To do this you would intercept each word at the application level and modify words with "ñ" or "n." That part of the query might look something like this:
<elem>quincea<oneof><choice>ñ</choice><choice>n</choice></oneof>era</elem>If you have to support this particular feature, neither solution is very palatable.
UPDATE 1: If you try and pass the choice operator to the Textml QueryAnalyzer object in .NET, it will throw an exception. quincea[n,ñ]era as a search string will fail. I've submitted this bug to Ixiasoft and I'm waiting for a response.
UPDATE 2: Ixiasoft has responded that this is a bug and is due to be released in their newest package, 18.104.22.1682.