First - there is no such thing as a 'standard' format, and every drive has to be (partitioned and) formatted before use - most operating systems use different (and frequently incompatible) file system formats, Linux has ext2, ext3 & rieser (to name a few), Windows has FAT, FAT32, and several different variations of NTFS.
Your question really becomes can I insert a (specify file system here) formatted drive into the DNS-323 (and use it) without formatting, and the answer may vary depending on what file system the drive was originally (partitioned and) formatted with. It will also depend on what drive configuration your DNS-323 is configured for.
If you insert a drive with the correct ext2 partition structure into a DNS-323 that is configured for standard volumes it will not prompt you to format it, and you will be able to access the files - to approach from a slightly different angle, if you were to place the same drive in a DNS-323 that is configured for RAID1 it will prompt you to format.
You can take a drive from the DNS-323 and connect it to a Windows system and you should be able to access it using an installable file system driver such as the ext2ifs - I have done this with Windows XP, I have heard of people having difficulty doing it with Vista, but I have not tried it with Vista personally.
One last thing - be very careful when swapping disks in and out of the DNS-323 - you just might create unneccessary headaches for yourself.