Convert rails app to radiant extension

I have seen a few people post something to the effect of, “it is simple
to convert an existing rails app into a radiant extension,” but I
haven’t been able to find anything that shows how to do this. Any
suggestions on where to look for this information?

I am trying to figure out what I can and can’t do with a rails app in
the context of an extension.

For example,

I took a look at the mailer extension and it looks they do not send
e-mails the way I know how – create a model that is based on
ActionMailer, create a template, and somewhere (in an observer or
controller) call MyMailerClass.deliver_my_mailer_method(my_object).

Is it not possible to send e-mail the way I described above?