Line-ups are sometimes still misunderstood as the generations-old action of the police rounding up a few usual suspects and ringers then letting a victim or witness view them and point one out. (Not saying this was your understanding, just clarifying that this old view exists but is erroneous.)
Modern line-ups are almost always done as photo line-ups. I think I know who did it. I request a phot line up. Clerk pulls up the DMV license photo of my suspect plus five others who look sorta like my suspect. The six photos are placed in a folder which labels them 1,2,3,4,5,6. I show the folder to my witness/victim who then sees if he/she can ID the suspect.
So, certainly I can have a suspect who's a sex offender in my line-up and certainly a sex offender can be a ringer (non-suspect) in my line-up.