I think this could work:

template<typename T>
DataList & operator+=(DataList &lhs, T &&rhs)
{
  lhs.reserve(lhs.size()+rhs.size());
  for(auto &&data : rhs)
  {
   if constexpr (std::is_rvalue_reference_v<T>)
     lhs.emplace_back(std::move(data));
   else lhs.emplace_back(data);
  }
  return lhs;
}