@Bruno Berlanda Tente assim:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<httpErrors errorMode="Custom">
<remove statusCode="404" subStatusCode="-1" />
<error statusCode="404" prefixLanguageFilePath="" path="/404.html" responseMode="ExecuteURL" />
</httpErrors>
<rewrite>
<rules>
<rule name="NameRule" stopProcessing="true">
<match url="(.*)" />
<conditions>
<add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
<add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
</conditions>
<action type="Rewrite" url="{R:1}.php" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>
Tuas páginas são php mesmo certo?
Att.