Skyline can automatically add [M+1], [M+2], etc isotopes to your Targets tree if you provide molecular formulas in your transition lists. Here are some things to know about that.
When you first import a transition list, Skyline will only show the most abundant isotope for precursors in the Targets window.
If you want to see others, there are a few settings that need to be adjusted in Transition Settings (via the Skyline Settings > Transition Settings... menu):
Note: in Skyline versions 21.2 and earlier, even if you have these values preserved in your settings from a previous run you may have to visit the Transition Settings and make a slight change to your settings to provoke the creation of the additional precursor nodes in the Targets window. It doesn't matter which setting, and you can change it back again afterward, but this is necessary to reevaluate the contents of the Targets tree.