Can you try with below sample code, may be this can help:
// create truststore
ResourceResolver adminResolver = getResourceResolver(ADMIN, ADMIN);
keyStoreService.createTrustStore(adminResolver, STORE_PASSWD.toCharArray());
KeyStore trustStore = keyStoreService.getTrustStore(adminResolver);
assertNotNull(trustStore);
InputStream is = ResourceReader.getResourceAsStream("/root-ca.crt");
CertificateFactory cf = CertificateFactory.getInstance(GraniteSecurityProvider.X509);
Certificate certificate = cf.generateCertificate(is);
is.close();
trustStore.setCertificateEntry("root_ca", certificate);
adminResolver.close();
Thanks!!