First, show the equation for the trendline, via Format Trendline->Display Equation on Chart
Then convert the shown equation to a formula, so that it will produce a Y for any given X.
Note though, your original data is probably on both sides of the trendline (i.e. the trendline is an estimated function), your new data will all be exactly on the new trendline (as it's actually generated from a known function).