mount ?
 

can i mount my device on a mount directory only?

or can i mount it under any /etc or /var or /usr.

is it allowed?

Mounting anywhere
 

Yes it is allowed :).
Just do: Where:
FILE_SYSTEM is file system you use on that media (vfat, ntfs, reiserfs, xfs, etc...)
DEVICE is a device you want to mount (like /dev/hda1 for first ide disk first partition)
MOUNTPOINT is a directory where you want to mount (like /var/backup)
Just make sure directory exists, before mounting.

Hope this helps :) !

You can mount a device anywhere it just doesn't work as well or isn't a standard procedure to mount them anywhere but under /mnt or a directory you create for mounting them.

mounting under /etc or /var should not damage my sys.

Whatever you do, don't mess around with /bin, /etc, /lib and /sbin because they contain essential system packages and config files, so mounting stuff in there or tampering with them can be potentially disastrous. Also /bin, /etc, /lib, /sbin and /tmp should always be on the root (/) filesystem, anything else can be mounted anywhere but /mnt or /media are good places to create mount points.