Have you ever searched scp manual for ways to append a file using it? scp is an excellent command to rapidly copy files and directories to / from a remote location. Unfortunately it is of no help when you just want to append a file over ssh connection. Here is a simple solution to append a file over ssh connection in either direction:

How to append remote file with local file over ssh connection

cat local-source-file-name | ssh user@some.domain "cat >> remote-target-file-name"

How to append to local file from remote file over ssh connection

ssh user@some.domain "cat remote-source-file-name" >> local-target-file-name