RE: Entities not Appearing in Process Steps
Sorry not an amazingly helpful answer but there was a article a few months ago asking the same and you cant do it.
I was looking at the out of the box actions to see if there was a way to do using them, they had the ability to do a quote line I believe but not the contract so that's not going to help, I tried making my own action to do it which may be a way forward but was not having perfect results with that but, it may be worth exploring.
Otherwise, do you specifically want to use the OOB Contract Line? Otherwise you could create a custom entity to hold the same information then problem solved you will be able to do via your workflow.