A shotgun that doesn't take interchangeable choke tubes is either fixed choke or no choke, depending on its purpose. It's just a question of whether or not the barrel narrows down near the end. Older trap shotguns are often fixed full choke: the barrel narrows down quite a bit. An 18" barrel like yours is typically on a home-defense gun, and will most likely have no choke (also called "cylinder bore").
This means that your shot pattern will spread out pretty quickly. On trap, this will hinder you on the longer shots: by the time the shot gets "way out there," it will have spread out so much that it could potentially pass around the bird without any pellets striking the bird. On the plus side, you have a 12-gauge, not a 20, so you've got a bigger shot cloud to begin with.
My advice it to get on the bird fast, and take your shot before it gets too far out. I've seen people do reasonably well with this strategy when shooting a cylinder-bore shotgun at trap.
EDIT TO ADD: H&R's website shows accessory barrels for sale for the Pardner. A fixed full choke barrel for the Pardner is $55 for a 28" and $75 for a 32". If you're planning to shoot a lot of trap, you might want to consider one of these. A 28" should be fine.
http://www.hr1871.com/Support/accessoryProgram.asp
They also have a 28" fixed modified, which is a more open choke than full. Full is good, but it can be harder to hit the close birds because without a lot of shot spread, your aim has to be more precise. The modified is a good compromise choke. A fixed modified 28" would give you a good all-around hunting/clayshooting set-up for your Pardner. You can put your 18" back on when you get home from the field/range.