Unless you have a land area for laying eggs, you probably don't want to take on more turtles. Even females not exposed to males can lay non-fertile eggs, and could develop egg-binding, which can be fatal if not treated. Some females will lay in the water, and some will not. If you want to breed, try getting a few painteds or muds, and breed turtles that, in the eyes of the average public, make better pets.
If you expand your pond to include land, make sure the area/yard is escape-proof!
If your pond is large enough to accomodate more turtles (how big is the pond?), then consider adopting another female slider rather than breeding.
Katrina