Not so much coil overs, more of a helper spring.
And be aware, they are generally considered a bad idea as you are now putting load through the top mounting and it could rip out without re-enforcement. The stock shocks just control the leaf spring without loading the sheet metal. Proper coil overs replace the leaf spring and the area around the top of the shock mount requires re-enforcement to do it.
There are replacement leaf springs available if you want a firmer ride, including composite mono leafs that allow a much higher spring rate than stock steel multi leafs, and are cheaper than a real coil over kit.