How to enter pinyin itself under Windows?
Usual IME's won't have that feature... so I think you have two approaches here.


Use a special IME or IME scheme, for example 地球拼音 from 中州韵输入法引擎(RIME). Reference: this Chinese post
Type Chinese first, then search for a Chinese-Pinyin conversion tool/online app, for example http://hanyu.iciba.com/pinyin.