ember-cli-mirage in legacy app

We have application that used Pretender to provide fixtures for tests. Now we're trying to migrate to ember-cli-mirage. We cannot migrate all the fixtures at once. So what is basically happening there is that we are starting our own Pretender server and ember-cli-mirage is starting it's own. Whic renders following warning:

You created a second Pretender instance while there was already one running. Running two Pretender servers at once will lead to unexpected results and will be removed entirely in a future major version.Please call .shutdown() on your instances when you no longer need them to respond.

Since it is just a warning, it should not be an issue for the transient period. Problem is that once Mirage is loaded into our application, old Pretender routes stops responding. I guess that's what "... will lead to unexpected results" is referring to.

Any chance to run ember-cli-mirage alongside manually created Pretender routes? Or just to use Mirage server and inject those routes there?