From your question, it sounds like what you want to do is to Migrate from a two drive RAID 0 to a RAID 4 drive RAID 10. RAID level Migration and virtual drive capacity expansion depend on what RAID stack / solution you are using. Before attempting to make any changes in your RAID configuration, you should backup all critical data.
If you are using Intel ESRTII Embedded RAID on and Intel Server Board, you have several migration options. If you are using Matrix RAID or OS level RAID, there are likely no options other than to backup your data and start over. My expertise is in regard to ESRTII Embedded RAID so I will direct my comments to that RAID solution.
If you are using Embedded RAID on an Intel Server Board, you can migrate from a two drive RAID 0 to a three or four drive RAID 5. There is not an option to migrate to a RAID 10 because of the way RAID 10 drives are designed (two RAID 1 mirrors with a RAID 0 striped across them). In order to migrate to a RAID 5, you must purchase and install the RAID 5 enablement key (AXXSRAK5). Instructions for migrating raid levels is available in the RAID SW Guide at: http://download.intel.com/support/motherboards/server/sb/d29305008_raid_swg.pdf.
These operations can be performed in either the BIOS Utiity or in the OS RAID Web Console 2 Utility.