Can you train a cat where to go outside?

We are new to cats, having taken in a stray about amonth ago.
She is an outdoor cat with a small house on the deck.

We have never seen her venture out...she seems to always come jumping
up from underneath the deck when we call her.

There is gravel underneath the deck, and we are worried that she may
be pooping under there. Mainly because we are being swarmed with
flies when we try to sit out on the deck. We don't smell anything
foul, though...we're just guessing.

Is there any way to train here to go to the bathroom somewhere
else....out in the cornfield, garden, etc., and insure that she
doesn't go under the deck?