You might have figured this out already (it's quite an old question), but I may as well answer it for anybody else who has the same question. The tags you use in include.xml are the same as in OpenFL's project.xml. What it does is basically add onto the project.xml.
So for example, this would be the include.xml to include your .ndll file:
<?xml version="1.0" encoding="utf-8"?>
<project>
<ndll name="hxssl" haxelib="hxssl" />
</project>
I haven't actually used this tag before myself, but I assume that name
is the .ndll filename without the extension, and haxelib
is the library where it is found.
You can find more info about the syntax and tags here.