Edit System Settings when installing a 3rd CMP

Hello everyone! I'm Francesco and I need your help!
I'm make a new 3rd CMP addon for a international web travel agency and I've made a new package that imports in modx a lot of resources,snippets,chunks,namespace and system settings/lexicon and many other object.
What I need is to edit core system settings during the install process so the content's editor people don't have to do that. (For example, the site_start resource's ID, that I need associate to a my resource that will be imported.)
I Don't understand where I can make the call to "$modx->getObject" for the editing of the settings. In the resolvers file? How? I try to do that but without results.