Yes, you can use the API for Email Marketer Pro to perform operations directly with Email Marketer Pro instead of doing so the control panel, such as importing users.
You can add contacts with a few simple commands, but if you wanted to use your existing list of users instead of adding them into Email Marketer Pro that can’t be done – you would have to import them in a batch using the API or Email Marketer Pro, or add them one-by-one with the API or from within Email Marketer Pro.