Using Huffman Encoding to improve Lempel-Ziv Encoding
Huffman encoding is already used this way.

Huffman encoding is already used together with LZ77 to improve its compression rate.  The most widely used instance of LZ77-style compression is DEFLATE.  DEFLATE combines LZ77 with Huffman encoding.  See https://en.wikipedia.org/wiki/LZ77_and_LZ78#Implementations and 
https://en.wikipedia.org/wiki/DEFLATE.

Huffman encoding is commonly used together with standard LZW (Lempel-Ziv-Welch) encoders to improve their compression rate.  See the Wikipedia article on LZW and other standard resources on LZW coding for details: https://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch