Encoding sample text-file into 20 symbol Huffman dictionary using MatLab

Hi, For a part of an assignment at uni I have to encode a sample text containing words, dates (in numbers), commas, dots and spaces. Only alphabet letters must be used for encoding in a 20 symbol dictionary size limit.

MatLab's build in Huffman-functions is preffered for solving this task. How do I limit the alphabet to 20 symbols and how would I go about this in general?

Thank you!