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To modify a TGA file and append content to a specific channel in C++, you would need to:

1. Read in the TGA file into a data buffer using file APIs like fopen(), fread(), etc.
2. Extract the header information of the TGA file from the data buffer.
3. Determine the data format of the TGA file (usually 24-bit or 32-bit).
4. Locate the desired channel in the data buffer and modify its content according to your needs.
5. Write the modified data buffer back to the TGA file using file APIs like fwrite(), fclose(), etc.

Here's some example code to get you started:

#include <cstdio>

int main()
{
    // Open the TGA file for reading
    FILE* tgaFile = fopen("myfile.tga", "rb");
    if (!tgaFile)
    {
        printf("Failed to open TGA file!\n");
        return 1;
    }

    // Read in the TGA file data
    fseek(tgaFile, 0, SEEK_END);
    const long fileSize = ftell(tgaFile);
    fseek(tgaFile, 0, SEEK_SET);
    unsigned char* imageData = new unsigned char[fileSize];
    const size_t bytesRead = fread(imageData, 1, fileSize, tgaFile);
    fclose(tgaFile);
    if (bytesRead != fileSize)
    {
        printf("Error reading TGA file data!\n");
        delete[] imageData;
        return 1;
    }

    // Extract the TGA header information
    const int width = imageData[12] + (imageData[13] << 8);
    const int height = imageData[14] + (imageData[15] << 8);
    const int bpp = imageData[16] >> 3;

    // Find the desired channel and modify its content
    const int channelIndex = 2; // For example, modify the blue channel
    for (int i = 0; i < width * height; ++i)
    {
        const int pixelIndex = i * bpp;
        imageData[pixelIndex + channelIndex] = 255; // Modify the channel value as needed
    }

    // Write the modified data buffer back to the TGA file
    tgaFile = fopen("myfile_modified.tga", "wb");
    if (!tgaFile)
    {
        printf("Failed to open TGA file for writing!\n");
        delete[] imageData;
        return 1;
    }
    const size_t bytesWritten = fwrite(imageData, 1, fileSize, tgaFile);
    fclose(tgaFile);
    if (bytesWritten != fileSize)
    {
        printf("Error writing modified TGA file data!\n");
        delete[] imageData;
        return 1;
    }

    // Cleanup
    delete[] imageData;

    return 0;
}

Note that this is just a basic example and assumes a lot of things about the TGA file format. You'll need to add error checking, handle different TGA formats, and modify the channel values appropriately for your specific use case.