Can carriers (operators) impose additional requirements for dangerous goods shipments?
Carriers (operators) are generally private companies and can impose just about any additional restrictions on dangerous goods shipments they want as a matter of their company policy.
In fact, in IATA Section 2.8.5, State (country) & Operator (airline) Variations, provides a place for governments and operators to publish these rules for the air shipping community to reference. The only caveat is that such variations must be MORE restrictive than the appropriate government regulation normally applicable to that particular material or shipping scenario.
However, whether or not an operator has published such requirements does not matter, airlines can pretty much create any policies they want as a condition of carriage for shipping dangerous goods with them.