If your instance-generating function is defined in an external module, you can use it in GHCi like this:

-- Enable TH
> :set -XTemplateHaskell 

-- Import modules (e.g. Aeson)
> import Data.Aeson.TH
> import Data.Aeson

-- Define the type for which I'm going to generate instances
> data X = X {x :: Int}

-- Derive the instance
Prelude Data.Aeson.TH> data Dummy; deriveJSON defaultOptions ''X

-- Check that it works
> toJSON (X 1)
Object (fromList [("x",Number 1.0)])

(More about the data Dummy trick here: https://www.reddit.com/r/haskelltil/comments/3ghacj/you_can_use_template_haskell_functions_like/.)