Is there a terminal command to enable Continuity - handoff on unsupported OS X?
It takes a bit more than just a Terminal command, but check out Continuity Activation Tool:


  This tool makes the necessary changes to enable OS X 10.10 and 10.11 Continuity features on compatible hardware. Continuity features activated by this tool include Handoff, Instant Hotspot, and New Airdrop. Support for OS X 10.11 (El Capitan) is currently experimental and does not support dongles yet.