FlexFabric Embedded Ethernet(LOM) is mapped to Interconnect Bays 1 & 2. I assume you are using GbE2c Ethernet Blade Switches, which is a 1Gb switch.
1. Nimble doesn't support FCOE.
2. You would need to buy NC542m/554m mezzanine cards with Virtual Connect Flex-10/10D Interconnect Bays and connect the Flex-10 Interconnect Bays 3 & 4 to a 10Gb switch. We use Brocade VDX6740 Switches for our iSCSI network.