With this script you can use the scp command.
Somehow there is a wrong path included in the scp binary.
This script creates a workaround by making a symbolic link in the directory where the scp binary expects the ssh binary:

qnap[~] # ls -l /usr/local/bin/
lrwxrwxrwx 1 admin administ 12 Nov 4 14:45 ssh -> /usr/bin/ssh*

The script consists of the following lines:
==========================
#!/bin/sh
# make scp work in qnap
#
mkdir -p /usr/local/bin
rm -f /usr/local/bin/ssh
ln -s /usr/bin/ssh /usr/local/bin/ssh
==========================

Instructions:
1) download scp.tar [10 KB]
2) move scp.tar to /share/MD0_DATA/custom
3) tar xf scp.tar
4) add the line /share/MD0_DATA/custom/scripts/scp.sh to /share/MD0_DATA/custom/autorunmaster.sh
5) execute /share/MD0_DATA/custom/autorunmaster.sh