Create a new sectionGroup in configSections called applicationSettings and paste your app.config configuration into web.config as shown below and then you can override your app.config settings.
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<sectionGroup name="applicationSettings"
type="System.Configuration.ApplicationSettingsGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" >
<section name="Playground.ConfigurationOverride.DataAccess.Properties.Settings"
type="System.Configuration.ClientSettingsSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
requirePermission="false" />
</sectionGroup>
</configSections>
<applicationSettings>
<Playground.ConfigurationOverride.DataAccess.Properties.Settings>
<setting name="MySetting" serializeAs="String">
<value>Setting in DataAccess</value>
</setting>
</Playground.ConfigurationOverride.DataAccess.Properties.Settings>
</applicationSettings>
</configuration>