It is possible, though it will require some coding from your end.

option 1: load at run-time

Basically you would register a function with the library something like this:

ref @load_function(const string &in library, const string &in function);

The function could be implemented to load the dll, find the function address of the cat function, then register the function with the engine and finally return the asIScriptFunction for the registered function in a CScriptHandle add-on.

In the script it would receive this reference and cast it to the appropriate funcdef before calling it. For example:

funcdef string FUNC(const string &in)
FUNC @cat = cast<FUNC>(load_function("cat.dll", “string cat(const string &in)”);

option 2: load at compile time

The CScriptBuilder add-on allows you to implement callbacks for #pragma directives. You could use this to implement a pragma directive that would load the cat dll and register the function with the engine before the script is compiled so it could be used just as any other function. For example:

#pragma load_function("cat.dll", “string cat(const string &in)”)

You can use asGetActiveContext to obtain the current context, and from that get the engine.

I haven't compiled this, but you should be able to do something like this:

// registered as 'ref @load_function(const string &in library, const string &in function)'
CScriptHandle load_function(const std::string &library, const std::string &funcSignature)
{
   asIScriptContext *ctx = asGetActiveContext();
   asIScriptEngine *engine = ctx->GetEngine();
   
   ... load library and get the function address
   
   int funcId = engine->RegisterGlobalFunction(funcSignature, ...);
   asIScriptFunction *func = engine->GetFunctionById(funcId);
   
   CScriptHandle ref;
   ref.Set(func, func->GetObjectType());
   return ref;
}

@witchlord I wanted to try this out too, my problem is that I have multiple scripts running that can register a dll function, but these scripts can be started and stopped dynamically, so when they are stopped the dll function should be unregistered. Is it somehow possible to unregister a previously registered global function?

Yes, that is also possible by using configuration groups. These can be removed as long as there are no scripts still referring to the registered functions.