Thank you both very much for such a quick reply.
Follow the "complete" code of what I have until the moment (there's still much to do).
About returning a void pointer, I've just did the same as the thread pool example in the Get Programming with QNX 
Neutrino guide does.

#define CAN_SERVER_PATH "/net/..."
//****************************************************************************
//  Global variables
//****************************************************************************



//****************************************************************************
//  Thread pool lifecycle functions
//****************************************************************************
//  Context manipulation functions when thread is created and killed
THREAD_POOL_PARAM_T * Comm::ContextAlloc(THREAD_POOL_HANDLE_T *handle)
{
    //  Stablish connection to CAN server
    CommServerStruct *comm_server = new CommServerStruct;
    comm_server->can_coid = open(CAN_SERVER_PATH, O_WRONLY);

    if(comm_server->can_coid == -1)
    {
        //  Failed to connect to CAN server
        //  Return error
    }
    //  Initialize values;
    comm_server->comm_rcvid = -1;

    return ((void*)comm_server);
}

void Comm::ContextFree(THREAD_POOL_PARAM_T *param)
{
    CommServerStruct *comm_server = (CommServerStruct *)param;
    delete comm_server;
}

//  Function to block waiting for a message from client
THREAD_POOL_PARAM_T * Comm::WaitForMessage(THREAD_POOL_PARAM_T *ctp)
{
    //  Cast pointer
    CommServerStruct *comm_server = (CommServerStruct *)ctp;
    //  Blocks waiting for message
    MsgReceive(comm_chid_, comm_server->msg, sizeof(comm_server->msg), NULL);

    return ((void*)comm_server); 
}

//  Message parsing function when unblocks with received message
int Comm::ParseMessage(THREAD_POOL_PARAM_T *ctp)
{
    //  Cast pointer
    CommServerStruct *comm_server = (CommServerStruct *)ctp;
    char return_message[16];
    //  Pass the message over to CAN server
    MsgSend(comm_server->can_coid, comm_server->msg, sizeof(comm_server->msg), return_message, sizeof(return_message));
    return 1;
}

//****************************************************************************
//  Communications set up and start
//****************************************************************************
int Comm::InitComm()
{
    if((comm_chid_ = ChannelCreate(0)) == -1)
    {
        //  Could not create Channel, insert error code
        return -1;

    }

    //  Set up the thread parameters for the threads that will populate the pool
    pthread_attr_t *t_attr;
    pthread_attr_init (t_attr);


    //  Set up the thread pool parameters
    thread_pool_attr_t tp_attr;
    thread_pool_t    *tpp;
    memset(&tp_attr, 0, sizeof (tp_attr));
    //tp_attr.handle = (void *) 0x12345678;  // passed to contextalloc
    tp_attr.block_func = WaitForMessage;
    tp_attr.unblock_func = UnblockFunc;
    tp_attr.context_alloc = ContextAlloc;
    tp_attr.handler_func = ParseMessage;
    tp_attr.context_free = ContextFree;
    tp_attr.lo_water = 2;
    tp_attr.hi_water = 6;
    tp_attr.increment = 1;
    tp_attr.maximum = 10;
    tp_attr.attr = t_attr;



    //  Create a thread pool that returns
    tpp = thread_pool_create (&tp_attr, 0);
    if (tpp == NULL)
    {
        //  Could not create thread pool
        return -1;
    }

    if (thread_pool_start(tpp) == -1)
    {
        //  Could not start thread pool
        ChannelDestroy(comm_chid_);
        //sleep (3000);
        return -1;
    }

    //  If it gets here, thread pool is up and running
    return 1;
}

And yes, I did assume it was more of a language problem.
QNX base it's guides and manuals all in C language.

Could you please give me some directions on how to deal with this C++ Method vs C function callback assignment?

Sorry, a bit of repeated comments, as I'm still filling up the whole thing, but got stuck in those initial issues.

Thank you very much for the help anyway!