Replace mixed content in XML (plain text + tags)

The program takes an xml that is mixture between html and custom tags. I want to replace this custom tag:

<replaceable />

With this:

Some text
<div>Other text</div>

The main problem is that any framework or library that I can use to properly handle XML (like LinqToXML or System.Xml) cannot get the whole content. If try like this:

// replaceable is an XElement for <replaceable />
// content is a string
replaceable.ReplaceWith(content)

It ends up all as escaped text, and is taken as the value for the XElement.

And if I try the XElement.Parse() method, like other questions here suggest, I lose the plain text at the beginning, and only the <div> remains.

Is there a way to do this kind of replacement without going all regex on it?