Is it possible to use the SdCard in portable mode but encrypt it?
It is not possible using the Android encryption of Marshmallow. Disk Encryption encompasses internal and external SD in the case of adoptable-storage and only internal SD in the case of portable storage ( since the external SD is considered a part of SD in the former)

Full Disk Encryption mentions other possibilities of encryption and does not cover what you wish to achieve. 

You may need to use a third party app, like SSE universal encryption, or other apps to encrypt files on external SD. I haven't tried this app but mentioning being open source and hence perhaps better