How to force Samba to create directory

You can use the preexec or root preexec options for this. They specify a script that is run upon connection to a share. In case of preexec the share is run as the connecting user, and as root with root preexec.

In your share:

[BACKUP]
root preexec = /etc/samba/gendir.sh %u

where /etc/samba/gendir.sh looks somewhat like this:

#!/bin/bash 
DIRECTORY=/storage/BACKUP/$1
if [ ! -d "$DIRECTORY" ]; then
   mkdir $DIRECTORY
fi

Depending on your requirements, add chown and/or chmod statements to the script.