Depending on the value, you might not even need a rider/endorsement to your current policy. But if you have a single cue over $2K, or even a collection over this amount, then you need the endorsement.
Adjusters are familiar with the fact the customers collect all kinds of things...but the key is documentation, and one reason I always get a receipt of any cue I buy, and also take good photos, and it always helps if you have a paid receipt for your purchase.
Nothing sends up a red flag then telling an adjuster you paid cash for something that cost $2500 or more and then can't remember the guys name you bought it from. I know that is how "we" do business in the pool world, but it ain't gonna fly in the insurance world.
Also, have your agent inspect your cues when he writes up your new endorsement because that will be the adjusters first phone call when you report them missing and thus your agent will be able to tell them not only did he write the endorsement but he personally inspected the cues., and that adds a lot of credibility to the claim process !
