Insert category on the Ribbon ---- way over to the right is the Symbols
sub-category. You should be able to find the schwa symbol in the Times New
Roman Font (or one of the other fonts) with the IPA extensions subset.
If it's for English pronunciation, you'll probably need a few other
phonetic characters. Most of them are in the Unicode range "IPA
extensions," but for Theta (for the th in "thin") go to the Greek
range, and for Edh (th in "this") go to Latin Extended-A. Between
those three ranges, you'll find everything you need for English.