Download the font[1] and put it in the '/koreader/fonts' directory[2].

1 https://font.download/font/bookerly

2. p13 https://koreader.rocks/koreader-user-guide.pdf

First, KOReader uses its own fonts in addition to whatever fonts are installed on your device, so yes. Second, you can find and download Bookerly on the internet and put it in the fonts folder, so also yes.