How to add key to global ssh_known_hosts without ssh-keyscan?

I want to add an entry to my /etc/ssh/ssh_known_hosts file without running ssh-keyscan. I tried adding the public key but ssh complains, that the fingerprint does not match.

From the sshd man page:

SSH_KNOWN_HOSTS FILE FORMAT

 The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host  public keys for all known hosts.  The global file should be prepared by  the administrator (optional), and the per-user file is maintained auto-  matically: whenever the user connects to an unknown host, its key is  added to the per-user file. 

So my take from that would be, that one can add the public key data. However, it doesn’t seem to work for me.

My ssh_known_hosts looks like this: (the key part matching the content of my public key file)

192.168.1.208 ssh-rsa AAAAB3Nza......zktpC1w== 

running ssh-keyscan gives:

192.168.1.208 ecdsa-sha2-nistp256 AAAAE2V...2L0= 192.168.1.208 ssh-rsa AAAAB3Nza...38Ll 192.168.1.208 ssh-ed25519 AAAAC3Nza...m2Sc 

So I can see, that the key shown for ssh-rsa ends differently from the content in my public key file.

How can I add the correct data to ssh_known_hosts file without ssh-keyscan?

(My environment: I use puttygen to create the keys, ssh-keygen etc. in a cygwin environment to convert as needed. Eventually the keys are used in Alpine linux VMs to ssh / scp between the VMs.)