This was actually discussed not by Liu or Bolland, but by Mark Hamill and Bruce Timm in an interview with Empire.
According to Hamill, he wanted them to adapt the graphic novel strictly as written, but he was told there simply wasn't enough material for a full-length feature:
Mark Hamill (actor, "The Joker"): I can't imagine how people are going to react to this, because I'm a Killing Joke purist. When they first talked about it, I said, "The only way we can do this is as a book on tape so that we honor every comma, every word, every letter, every syllable of Alan Moore's script. We can add music and special effects to enhance it." They kind of said, "What are you talking about? Nobody is doing this as a book on tape. It's not commercially viable for us to do it that way. See if you can get the rights and record it in your basement or something if that's what you want to do. This story has to be expanded." If we just adapted The Killing Joke as an animated film, it would maybe be fifty-five minutes. They've actually done a really incredible job of supplementing it with Barbara Gordon/Batgirl material.
Timm then explained that they chose to use Batgirl more extensively in the story both to feature a character who didn't get enough attention, and to make the climax of the story more affecting:
Bruce Timm (producer): We thought if we were going to expand this to feature length, we didn't want to just pad out the original story by putting in a bunch of stuff between sequences of the story, because it's literally a whole other half of movie that we could add. So we took that opportunity to basically tell a Batgirl story, which we don't often get a chance to do these days. And it was great, because we could spend more time with her as a character and get to understand what she's all about and how she's similar to Batman in some ways, and really different in others. They come at the crime fighting thing from two completely different places. The good side of that is we get to spend more time with her and learn that she's an interesting character. We get to really like her. The bad side of that is that we get to like her so much that when The Killing Joke part of the story happens, it's, like, "Oh, no!", because we really like her. So it's a double-edged sword.