If you look at the list of arguments of scatter.smooth(), it has an argument "..." which can be used to pass graphical parameters. So, you can use "main" to add the title to scatter.smooth() plot.
scatter.smooth(x, y = NULL, span = 2/3, degree = 1,
family = c("symmetric", "gaussian"),
xlab = NULL, ylab = NULL,
ylim = range(y, pred$y, na.rm = TRUE),
evaluation = 50, ..., lpars = list())
Here is an example to show how to use "main."
scatter.smooth(x, y, main="plot title here")