Can You Use the FX Smooth Twist Barrel with Slugs/

They'll shoot poorly

You can but accuracy won't be there. Get yourself a slug liner. It's only $110. If you can afford a 2k gun I don't see why you can't afford a $110 liner.

From my understanding, a slug liner will not fit in the original impact

Maybe try some FX Hybrid Slugs?

Hybrid slugs will shoot ok, but nowhere close to as accurately as pellets. We tried Hybrids last weekend in a .22 Bobcat and they were about 1 to 1.5 inch groups at 50 yards. Pellets (18.1) were 1/2 or less. FYI, the 25.4 RD Monsters were as accurate or even more so than the 18.1s even though we were shooting them at 715 FPS. 

Can't get ANY type of slug/hybrid to shoot well out of either of my original Smooth Twist barrels (FX Royale .25 & Boss .30).They're just not designed for them. Don't throw away money trying different types, they do not work. However, the Hades .25 shoot amazingly accurate from my Royale & I have zero concern now over not being able to shoot slugs. Hope MY real world experience saves you some time & money. Happy shooting! 

Only tried my 22 streamline and hybrids were the only one that would work. Get 1.5-2 inch groups at 100. Not super accurate but good enough for windy days when pellets have no chance. Otherwise I stick with pellets 

I have not had any luck with slugs out of my crown. Thinking of getting a slug liner myself.

$109 dollars for a slug liner is a very good investment.