Passing a generator extention through a wall for use in a power outage
You need a generator inlet  that feeds a single outlet.(you seem to know what an inlet is, but it doesn’t need to feed the breaker box, it could just feed a single outlet.) This new outlet should be placed next to your current sump pump outlet, and properly labeled to avoid future confusion.  (Assume here I’m talking about a future homeowner or tradesman— you won’t be confused but others might be in 10-20 years.) This new outlet would be dead when not fed by the generator. 

Do NOT run your generator in the garage, it must be outdoors! Generators emit Carbon Monoxide which is odorless and extremely lethal.

(Note: the linked example is weatherproof, non-weather proof models are made for standard knockout sizes the are found on Hoffman-style enclosures, which are typically larger than needed for this project.  Since the easiest to install are all weatherproof there is a message there—these should be indoors or in the garage.)