We do have an endpoint in our V2 API that allows you to upload contact data that is structured in a JSON format. In order to do this, you simply need to be able to send a POST request containing the JSON data as the body. You can read the details about this API endpoint here: http://developer.constantcontact.com/docs/bulk_activities_api/bulks-activities-import-contacts-api.h...
In addition, we do offer a set of Software Development Kits for use with Constant Contact's V2 API that support this functionality as well. You can find our SDKs located here: http://developer.constantcontact.com/libraries/libraries.html