Hi Ed, Welcome to the forum and to Band-in-a-Box.
One easy way to get a song riff into your song is to obtain a midi file of the song then import the midi into Band-in-a-Box. That brings up the question, where to find midi song files?
+++ HERE +++ is an article that lists eight websites that offer midi song files.
Another way is to use a program or application to separate the vocals and instruments in an audio file into audio stems. Then use a stem as a guide to recreate the performance.
+++ Audacity +++ is a free audio editor that has audio file split into stems technology.
I'm sure other forum members will share more ideas with you.