Hi Vasilis,
A Data Center has multiple pair of N5k Series switches in vPC setup. Multiple FEX N2k are connected to these switches. We don’t have available interfaces to the N5k and we have a pair of blade switches that are needed to get connected the soonest possible. Can we use the FEX to connect the blade switch?
Interesting and tricky question! Thank you Annette for giving us the chance to discuss about this 
Theoretically this is not possible but there is a good workaround for this to make it work!
The role of a Fabric Extender is to provide end-host connectivity into the network fabric. So, BPDU Guard is enabled on all its host interfaces and the interface state changes to error-disabled state when a BPDU is received e.g by connecting a switch. Besides, you cannot disable BPDU Guard on the host interfaces of the Fabric Extender.
BUT… If you do not run spanning tree on the switch you can connect the blade switch to the FEX.
So, you just need to disable the transmission of BPDUs from the switch by applying a bpdu filter or to use another technology for redundancy except of STP like port-channeling and Flexlinks. Because spanning tree is not used to eliminate loops, you should ensure a loop-free topology below the Fabric Extender host interfaces.
Let me know if you have any question