Import/Install SSL-certificate without CA-flag
Is it possible to install this self-signed certificate to Android without having a set CA-flag?


Android uses OpenSSL (or the fork BoringSSL in later versions) as the underlying SSL stack. OpenSSL will not accept a self-signed certificate unless the CA flag is true, because even a self-signed certificate must have the correct flags so that it can sign itself (i.e. CA flag). So even if you would be able to install the certificate it will not be used as the trust anchor for validation, i.e. the validation will fail.


  If not, is it possible to modify that existing certificate, so it can be installed?
  NOTE: Creating a new certificate is NOT a real option.


No, you cannot modify an existing certificate. You could probably re-create the certificate if you have access to the original certificate and the private key.