add additional hdd on existing raid 0...possible?

The only way this can be done is if your RAID controller supports Online Capacity Expansion (OCE). This then is a 2-step process:

1. Use OCE to expand the RAID volume size.
2. Use a partition tool or the Windows DISKPART command-line utility to expand the file system partition.

Yes, that's correct. Partition Magic will work, as will GParted. The Windows built-in DISKPART utility is what I use, although it's more limited in the operations it can perform.

Basically you will use one of these tools (see their documentation) to expand the NTFS file system partition on your RAID.

As an example of the whole process:

1. You have a 500GB RAID 0 (2x250GB drives). You want to add another 250GB drive (3x250GB = 750GB). Right now, Windows sees the RAID as a single 500GB drive with one 500GB NTFS partition on it.
2. You shut down the machine, plug the 3rd drive in, go into the RAID controller software and activate the Online Capacity Expansion so that the 3rd drive becomes part of the RAID array.
3. Once the OCE is complete, the RAID is now 3 drives and 750GB in size. The partition on that drive is still only 500GB. Windows will now see a 750GB drive with one 500GB partition on it and 250GB of free space.
4. You run the partition manager software. You choose the 500GB partition and tell the software to expand it to the maximum size. Now the partition is 750GB. Windows now sees one 750GB drive with one 750GB partition on it.

No, not all RAID cards are capable of OCE. You will need to go to the RAID card manufacturer's web site and look at the specifications for the RAID card to find out if it is supported and how to use it.

If the RAID card supports OCE, all data will remain intact during the entire expansion process.