Yes, It is possible, if use MultiValueMap headers instead of HttpHeaders
Example:
MultiValueMap<String, String> headers = new LinkedMultiValueMap<String, String>();
headers.add("Authorization", "Basic " + base64Creds);
headers.add("Content-Type", "application/json");
RestTemplate restTemplate = new RestTemplate();
restTemplate.getMessageConverters().add(new MappingJackson2HttpMessageConverter());
HttpEntity<ObjectToPass> request = new HttpEntity<ObjectToPass>(objectToPassInstance, headers);
restTemplate.postForObject(urlPost, request, Boolean.class);
Boolean.class just because my controller returns boolean at this endpoint (could be anything)
Good luck with coding!