Yes, you can.
Most of the functions in Strix Unreal SDK are provided as Blueprint functions, but you can call them from C++ codes, too.
The Blueprint functions of Strix Unreal SDK are defined in the UStrixBlueprintFunctionLibrary class. You can call these functions from C++, following the ordinary calling convention of the Blueprint functions. Also, most of the Blueprint functions have corresponding C++ API defined in the UStrixNetworkFacade class, and you can call them instead.
You can also define your custom messages and send and receive them via Message Relay API. Please refer to the Relay Message section in Strix Unreal SDK User’s Guide for details.