Is it possible to add more hard disks to RAID10 while keeping it online

I couldn't disagree with joeqwerty more to be honest. You don't mention which controller you have but all of the PERC 5 and 6 cards can perform live array expansion, although I'd be strongly tempted to fully backup your volume first. This way your new drive/s will become part of the R5 array and so will be able to survive a single disk failure. If you simply add another extent then in the event of losing a disk you stand a good chance of losing the whole VMFS datastore (the risk lessening if you're on ESX/i v4), plus expanding the array to 6 drives gives you 300GB more space than two 3 drive arrays.

Normally you can reconfigure your drives from a RAID5 to a RAID6 on the controller and it will not delete data. Then, you just have to extend the partition on the OS level.

It is still VERY RECOMMENDED to have a backup if something unexpected happen!