Something from xWebAdministration module should help you. Below are the DSCResources available. check xWebConfigProperty and xWebConfigPropertyCollection.
ImplementedAs Name ModuleName Version Properties
------------- ---- ---------- ------- ----------
PowerShell WebApplicationHandler xWebAdministration 2.4.0.0 {Name, Path, AllowPathInfo, DependsOn...}
PowerShell xIisFeatureDelegation xWebAdministration 2.4.0.0 {Filter, OverrideMode, Path, DependsOn...}
PowerShell xIisHandler xWebAdministration 2.4.0.0 {Ensure, Name, DependsOn, PsDscRunAsCredential}
PowerShell xIisLogging xWebAdministration 2.4.0.0 {LogPath, DependsOn, LogCustomFields, LogFlags...}
PowerShell xIisMimeTypeMapping xWebAdministration 2.4.0.0 {ConfigurationPath, Ensure, Extension, MimeType...
PowerShell xIisModule xWebAdministration 2.4.0.0 {Name, Path, RequestPath, Verb...}
PowerShell xSSLSettings xWebAdministration 2.4.0.0 {Bindings, Name, DependsOn, Ensure...}
PowerShell xWebApplication xWebAdministration 2.4.0.0 {Name, PhysicalPath, WebAppPool, Website...}
PowerShell xWebAppPool xWebAdministration 2.4.0.0 {Name, autoShutdownExe, autoShutdownParams, aut...
PowerShell xWebAppPoolDefaults xWebAdministration 2.4.0.0 {ApplyTo, DependsOn, IdentityType, ManagedRunti...
PowerShell xWebConfigKeyValue xWebAdministration 2.4.0.0 {ConfigSection, Key, WebsitePath, DependsOn...}
PowerShell xWebConfigProperty xWebAdministration 2.4.0.0 {Filter, PropertyName, WebsitePath, DependsOn...}
PowerShell xWebConfigPropertyCollection xWebAdministration 2.4.0.0 {CollectionName, Filter, ItemKeyName, ItemKeyVa...
PowerShell xWebsite xWebAdministration 2.4.0.0 {Name, ApplicationPool, ApplicationType, Authen...
PowerShell xWebSiteDefaults xWebAdministration 2.4.0.0 {ApplyTo, AllowSubDirConfig, DefaultApplication...
PowerShell xWebVirtualDirectory xWebAdministration 2.4.0.0 {Name, PhysicalPath, WebApplication, Website...}