How to use shared ItemGroup element in batching tasks - msbuild

Question

I'm trying to automate the creation of Firefox addon for two different platforms by using MSbuild:
I have shared files set which are the same for Mac and Windows and have platform specific files.
I want to batch the task of making XPI(which is just a renamed Zip file) by platform, but I can't find the right way to add the platform agnostic(shared) files as input for Zip task. Currently, my solution is to duplicate shared files items with platform windows and with platform mac, and then batch Zip task by Platform parameter. I have a feeling that my solution is not optimal. Maybe community can propose a better solution. Below is the simplified solution I created with comments:
<?xml version="1.0" encoding="utf-8"?>
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<PropertyGroup>
<ProjectMSBuildToolsPath Condition=" '$(ProjectMSBuildToolsPath)' == '' ">MSBuild</ProjectMSBuildToolsPath>
</PropertyGroup>
<!-- Required Import to use MSBuild Community Tasks -->
<Import Project="$(MSBuildExtensionsPath)\MSBuildCommunityTasks\MSBuild.Community.Tasks.Targets"/>
<!-- Default platform type is shared-->
<ItemDefinitionGroup>
<ZipFiles>
<Platform>Shared</Platform>
</ZipFiles>
</ItemDefinitionGroup>
<ItemGroup>
<ZipFiles Include="chrome\overlay.js" />
<ZipFiles Include="chrome\Win\methodContainer.js">
<Platform>Win</Platform>
</ZipFiles>
<ZipFiles Include="chrome\Mac\dataContainer.js">
<Platform>Mac</Platform>
</ZipFiles>
</ItemGroup>
<Target Name="_PrepareItemsForZip" Outputs="$(Platform)">
<ItemGroup>
<!-- Merge Shared and Windows specific files -->
<ZipFilesToWin Include="#(ZipFiles)" Condition="('%(ZipFiles.Platform)' == 'Shared') Or ('%(ZipFiles.Platform)' == 'Win')" >
<Platform>Win</Platform>
</ZipFilesToWin>
<!-- Merge Shared and Mac specific files -->
<ZipFilesToMac Include="#(ZipFiles)" Condition="('%(ZipFiles.Platform)' == 'Shared') Or ('%(ZipFiles.Platform)' == 'Mac')" >
<Platform>Mac</Platform>
</ZipFilesToMac>
</ItemGroup>
<!-- Merge Mac and Windows files set -->
<ItemGroup>
<_ZipFiles Include="#(ZipFilesToWin);#(ZipFilesToMac)" />
</ItemGroup>
</Target>
<!-- batch zipping files based on input platform -->
<Target Name="MakeXPI" DependsOnTargets="_PrepareItemsForZip" Inputs="#(_ZipFiles)" Outputs="%(Platform)" >
<Message Text="Zipped files: #(_ZipFiles) %(Platform)" Importance="high"/>
<Zip Files="#(_ZipFiles)" WorkingDirectory="" ZipFileName="CoolAddon-%(Platform).xpi" ZipLevel="9" />
</Target>
</Project>

Answer 1

Extract them to file like SharedProperties.properties:
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0"
xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<PropertyGroup>
<ZipFiles>
<Platform>Shared</Platform>
</ZipFiles>
<PropertyGroup>
</Project>
And then simply import in targets/script you need them:
<Project ... >
<Import Project="SharedProperties.properties" />