A biml script that should create a project level parameter named FolderBase, a package named POC which has a package level parameter, subfolder and a Variable that puts it all together.

ProjectParameter.xml

<Biml xmlns="http://schemas.varigence.com/biml.xsd">
    <PackageProjects>
        <PackageProject Name="ProjectParameterTest" ProtectionLevel="DontSaveSensitive">
            <Parameters>
                <!-- I think this is the project parameter-->
                <Parameter DataType="String" Name="FolderBase">C:\ssisdata</Parameter>
            </Parameters>
            <Packages>
                <Package IsEntryPoint="true" PackageName="POC"/>
            </Packages>
        </PackageProject>
    </PackageProjects>
    <Packages>
        <Package Name="POC" ConstraintMode="Linear">
            <!-- Package parameter-->
            <Parameters>
                <Parameter DataType="String" Name="Subfolder">POC</Parameter>
            </Parameters>
            <Variables>
                <Variable Name="FolderInput" DataType="String" EvaluateAsExpression="true">@[$Project::FolderBase] + "\\" + @[$Package::Subfolder]</Variable>
            </Variables>
        </Package>
    </Packages>
</Biml>