Yes, it is possible-but your Ops Manual must clearly state under which conditions the helicopter MUST be flown with two pilots....
The manufacturer states it is a Single-Pilot IFR certified helicopter, so you canīt use the RFM, but your Ops Manual can define it as "Multi Pilot".
This Ops Manual-however-must be approved by the authorities first.....
And-keep in mind:
This means your crews MUST be Multi-Pilot-Type Rated VFR and IFR on the EC135!
They will not be allowed to fly a single pilot 135 then, unless they have done a single-pilot type rating as well...