I assume you have the lyrics in a document file somewhere and want to use those. BIAB has something called Lyric documents where you can copy lyrics from an external file into your song. Once there, you can use the Big Lyrics window to read them - like Karaoke. This is not the same as having a lead sheet which is uses note-based lyrics. This has to be manually done although you can use copy/past somewhat.

Check out the BIAB manual on pg 240 for detailed descriptions of these.