Hi Belen,
The HiFive1 Rev B doesn’t support the SiFive Custom Instruction Extension (SCIE). In fact none of our hardware platforms support it. SCIE is a feature that is tightly integrated into the processor core. When it is enabled, the custom instructions need to be defined at design time. So even if HiFive1 did support SCIE, you wouldn’t be able to add your own custom instructions. You would only be able to use the new instructions that were already designed in.
We have a white paper on SCIE that you might find interesting in our Support Portal: