Have a look at the article
Authoring a Custom Bootstrapper Package for Visual Studio 2005
If you locate a folder
C:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\BootStrapper\Packages (VS 2005)
or, for VS 2008,
C:\Program Files\Microsoft SDKs\Windows\v6.0A\Bootstrapper\Packages
Each folder under 'Packages' is the prerequisite you see in the list, as shown on your screenshot.
So, if you want to add an application called MyPrereq as a prerequisite, you need to make your own folder 'MyPrereq' under 'Packages'. Then you make a product.xml file similar to this
<?xml version="1.0" encoding="utf-8"?>
<Product ProductCode="MyPrereq" xmlns="http://schemas.microsoft.com/developer/2004/01/bootstrapper">
<PackageFiles CopyAllPackageFiles="false">
<PackageFile Name="MyPrereq.exe" />
</PackageFiles>
<InstallChecks>
</InstallChecks>
<Commands Reboot="None">
<Command PackageFile="MyPrereq.exe" EstimatedInstallSeconds="90">
<InstallConditions>
</InstallConditions>
<ExitCodes>
<ExitCode Value="0" Result="Success"/>
<DefaultExitCode Result="Fail" String="GeneralFailure" FormatMessageFromSystem="true" />
</ExitCodes>
</Command>
</Commands>
</Product>
and your package.xml file similar to this
<?xml version="1.0" encoding="utf-8"?>
<Package Name="MyPrereq" Culture="Culture" xmlns="http://schemas.microsoft.com/developer/2004/01/bootstrapper">
<Strings>
<String Name="Culture">en</String>
<String Name="DisplayName">MyPrereq</String>
<String Name="GeneralFailure">A fatal error occurred. The installation failed.</String>
</Strings>
</Package>
and place these files and your setup package (MyPrereq.exe) in the folder. Check the existing packages as an example to see where to place files.
If you do everything properly, you will be able to see your MyPrereq option in the list of "Choose which prerequisites to install".