The RadFlowDocumentEditor in fact does not support navigation, so it wouldn't be able to add text in the middle of a span for example. The editor maintains an internal position inside the document, which points either inside a paragraph (to an inline) or directly after the end of a table element. In order to change the position in the document you could use some of the RadFlowDocumentEditor's methods, which are listed in this help article
To insert content at the end of the document, you could find the last InlineBase element with the generic EnumerateChildrentOfType method and move the position using MoveToInlineEnd():
InlineBase lastInline = editor.Document.EnumerateChildrenOfType<InlineBase>().Last();
If you are trying to append several documents together you can also take advantage of the Merge()
Hope this helps.