LWIP pbuf copy issue

Question

I am using the LWIP library on my stm32f4 project. In the low_level_output function have pbuf p member. I need to change its length with length + 1 and I need to add 1 byte end of the pbuf->payload but I dont want to change pbuf so I need to define new pbuf to copy this pbuf contents, than I need to use this line for(q = mypBuf; q != NULL; q = q->next)instead of below for loop. how can I do this ? Please help me.

static err_t low_level_output(struct netif *netif, struct pbuf *p)
{
  err_t errval;
  struct pbuf *q;
  uint8_t *buffer = (uint8_t *)(heth.TxDesc->Buffer1Addr);
  __IO ETH_DMADescTypeDef *DmaTxDesc;
  uint32_t framelength = 0;
  uint32_t bufferoffset = 0;
  uint32_t byteslefttocopy = 0;
  uint32_t payloadoffset = 0;
  DmaTxDesc = heth.TxDesc;
  bufferoffset = 0;

  /* copy frame from pbufs to driver buffers */
  for(q = p; q != NULL; q = q->next)
    {
      /* Is this buffer available? If not, goto error */
      if((DmaTxDesc->Status & ETH_DMATXDESC_OWN) != (uint32_t)RESET)
      {
        errval = ERR_USE;
        goto error;
      }

      /* Get bytes in current lwIP buffer */
      byteslefttocopy = q->len;
      payloadoffset = 0;

      /* Check if the length of data to copy is bigger than Tx buffer size*/
      while( (byteslefttocopy + bufferoffset) > ETH_TX_BUF_SIZE )
      {
        /* Copy data to Tx buffer*/
        memcpy( (uint8_t*)((uint8_t*)buffer + bufferoffset), (uint8_t*)((uint8_t*)q->payload + payloadoffset), (ETH_TX_BUF_SIZE - bufferoffset) );

        /* Point to next descriptor */
        DmaTxDesc = (ETH_DMADescTypeDef *)(DmaTxDesc->Buffer2NextDescAddr);

        /* Check if the buffer is available */
        if((DmaTxDesc->Status & ETH_DMATXDESC_OWN) != (uint32_t)RESET)
        {
          errval = ERR_USE;
          goto error;
        }

        buffer = (uint8_t *)(DmaTxDesc->Buffer1Addr);

        byteslefttocopy = byteslefttocopy - (ETH_TX_BUF_SIZE - bufferoffset);
        payloadoffset = payloadoffset + (ETH_TX_BUF_SIZE - bufferoffset);
        framelength = framelength + (ETH_TX_BUF_SIZE - bufferoffset);
        bufferoffset = 0;
      }

      /* Copy the remaining bytes */
      memcpy( (uint8_t*)((uint8_t*)buffer + bufferoffset), (uint8_t*)((uint8_t*)q->payload + payloadoffset), byteslefttocopy );
      bufferoffset = bufferoffset + byteslefttocopy;
      framelength = framelength + byteslefttocopy;
    }

  /* Prepare transmit descriptors to give to DMA */
  HAL_ETH_TransmitFrame(&heth, framelength);

  errval = ERR_OK;

error:

  /* When Transmit Underflow flag is set, clear it and issue a Transmit Poll Demand to resume transmission */
  if ((heth.Instance->DMASR & ETH_DMASR_TUS) != (uint32_t)RESET)
  {
    /* Clear TUS ETHERNET DMA flag */
    heth.Instance->DMASR = ETH_DMASR_TUS;

    /* Resume DMA transmission*/
    heth.Instance->DMATPDR = 0;
  }
  return errval;
}