The easiest and the accurate way to build a RAID 10 from a RAID 1 will be creating an image of the whole hard drive and save it on an external drive. I recommend you to use 3rd party software (like Symantic ghost) to create the image.
Once you have all you information backup on that image, you can format the hard drives and then save the image on one drive and create the RAID on Windows.
Remember to set the SATA mode to RAID before to save the information again.
I would also recommend you to contact Dell, and ask if the chipset of you system supports RAID migrations.