And in the UK, I(ntrinsically) S(afe) circuits would typically use blue sheathed cable, terminal blocks, etc, to clearly differentiate them from non-IS circuits (which would be suitably segregated from IS).
If your existing I/O is indeed Intrinsically Safe, then any new I/O in the same field area needs to be IS (unless the zone classification has changed). As has been said, Intrinsically Safe barriers are used to provide protection to allow electrical equipment to be used in Hazardous areas - modification to such a system should not be made without a good knowledge of the area, system and protection concepts,
regards
Dave