How to insert to std::map which is in std::multimap?

How can I insert another map into the map?

In the code, I try to copy the map from another.

multimap<string, map<size_t, size_t>> sorted;

for (auto itr = m_Items.begin(); itr != m_Items.end(); ++itr)
    sorted.emplace(itr->first
        , make_pair(itr->second.m_Date.m_Time, itr->second.m_Cnt)
    );