diff --git a/src/Falco/ConfigBuilder.fs b/src/Falco/ConfigBuilder.fs
index ab7757e..c38b381 100644
--- a/src/Falco/ConfigBuilder.fs
+++ b/src/Falco/ConfigBuilder.fs
@@ -9,14 +9,16 @@ type ConfigFile =
     | JsonFile of path : string
 
 type ConfigBuilderSpec =
-    { AddEnvVars    : bool
+    { Config        : IConfigurationBuilder -> IConfigurationBuilder
+      AddEnvVars    : bool
       BasePath      : string
       RequiredFiles : ConfigFile list
       OptionalFiles : ConfigFile list
       InMemory      : Map<string, string> }
 
     static member Empty =
-        { AddEnvVars    = false
+        { Config        = id
+          AddEnvVars    = false
           BasePath      = Directory.GetCurrentDirectory()
           RequiredFiles = []
           OptionalFiles = []
@@ -29,6 +31,8 @@ type ConfigBuilder (args : string[]) =
     member _.Run(conf : ConfigBuilderSpec) =
         let mutable bldr = ConfigurationBuilder().SetBasePath(conf.BasePath)
 
+        conf.Config bldr |> ignore
+
         for file in conf.RequiredFiles do
             match file with
             | IniFile file  -> bldr.AddIniFile(file, optional = false, reloadOnChange = true)
@@ -53,6 +57,10 @@ type ConfigBuilder (args : string[]) =
 
         bldr.Build() :> IConfiguration
 
+    [<CustomOperation("config")>]
+    member _.Config (conf : ConfigBuilderSpec, fn : IConfigurationBuilder -> IConfigurationBuilder) =
+        { conf with Config = fn }
+
     /// Sets the base path of the ConfigurationBuilder.
     [<CustomOperation("base_path")>]
     member _.SetBasePath (conf : ConfigBuilderSpec, basePath : string) =