Can drush make fetch dependencies for a newly added module, without using it to build the whole site? If a module includes a drush make file to fetch its dependencies, is it possible to run that make file by itself somehow against a pre-existing site's codebase? In other words, I didn't use drush make to build the site, but I'd like to use it just to fetch the new module's dependencies. Is that possible?
Yes, it is possible. Just run the makefile on the current codebase, and specify --no-core option from the drupal root:

$ drush make --no-core sites/all/modules/mailchimp/mailchimp.make.example


Here's the log of a test I did:

$ drush dl drupal
$ cd drupal-7.28/
$ drush dl mailchimp
  Project mailchimp (7.x-3.1) downloaded to /tmp/drupal-7.28/sites/all/modules/mailchimp.

$ drush make --no-core sites/all/modules/mailchimp/mailchimp.make.example
  Make new site in the current directory? (y/n): y
  Beginning to build sites/all/modules/mailchimp/mailchimp.make.example.
  libraries-7.x-2.2 downloaded.
  mailchimp downloaded from https://bitbucket.org/mailchimp/mailchimp-api-php/get/2.0.4.zip.

$ ls sites/all/libraries/
  mailchimp
$ ls sites/all/modules/
  libraries  mailchimp  README.txt


If any of the projects/libraries declared in the makefile already exists, it will be overwritten.