class AccessToken implements AccessTokenEntityInterface
{
    // ...

    private $userRole;

    public function setUserRole($role) { $this->userRole = $role }
    public function getUserRole() { return $this->userRole }

    private function convertToJWT()
    {
        return $this->jwtConfiguration->builder()
            // ...
            ->withClaim('role', $this->getUserRole())
            // ...
    }
}

class AccessTokenRepository implements AccessTokenRepositoryInterface
{
    // ...

    public function getNewToken(ClientEntityInterface $clientEntity, array $scopes, $userIdentifier = null)
    {
        $accessToken = new AccessTokenEntity();
        // ...

        if ($userIdentifier !== null) {
            $accessToken->setUserIdentifier($userIdentifier);

            $row = $this->db->query('SELECT role FROM users WHERE id = :id', ['id' => $userIdentifier]);
            $accessToken->setUserRole($row['role']);
        }

        return $accessToken;
    }

    // ...
}