From the Help section:

Your PBS Bookshelf is the list of books you have Posted to offer for swapping.  It also contains books you have received from PBS (these go "on hold" onto your bookshelf when you mark the books received, unless you choose not to do this at the time of marking the book received).

If I had a book that I received from PBS but later removed from my TBR pile, I can't add it back as an "on hold" book?  I just tried re-adding it to my TBR pile, then to my BIR, but it will not show up on my bookshelf.  I don't want to post it right now because it is wishlisted, but I'd like it on the bookshelf anyway.  From reading the Help Center paragraph above, I am assuming this just can't be done -- am I right?

Okay there is a way to do it but it's not really straight forward. 

1.) Place your account on "vacation hold"

2.) Post book

3.) find the book on your shelf and check the box

4.) select the hold button at the top of the screen

5.) Release the vacation hold from your account

the book should now be on hold on your shelf...

ETA: I did try this with a WL book and it worked. 

It worked!  Thanks, Kristen

you're welcome!