Note : This applies starting in Nightingale 1.11.x branch.
This is a quick overview about porting Songbird add-ons to Nightingale. It should work in most cases, as there are only minor API differences.
To get access to the add-ons data, just rename the .xpi file to .zip or directly open it with a ZIP-Archive program. You will see a few folders (defaults, chrome, skin or similar) and a few files on the top level. Only one file is relevant, the install.rdf. After you modified it according to the next section, remember to copy it back into the archive and rename it back to .xpi, if needed.
Inside the <Description> tag, add a compatibility section:
<!-- Nightingale --> <em:targetApplication> <Description> <em:id>email@example.com</em:id> <em:minVersion>1.11.0a</em:minVersion> <em:maxVersion>2.0a</em:maxVersion> </Description> </em:targetApplication>
We want the add-on to be compatible starting with the first 1.11 builds and up to 2.0a (alpha builds of the next nightingale, which does not yet build).
For more information about install.rdf, refer to this article.
You can put the add-on into a folder within your profile directory so you don't have to zip changes everytime, the Building an Extension Article describes the process a bit.