To best of my knowledge, there is no such feature in the Wix project schema. All files should be explicitly listed.
The WiX approach is different. It has the concept of "harvesting" of the data from directories, sites and other sources. This is done through the utility called "heat":
http://wixtoolset.org/documentation/manual/v3/overview/heat.html[
^].
Now, if this file, "Helper.dll", is your own component with source code, you should do completely different thing. You should include this project in the same solution as your WiX project and reference the "Helper" project by your WiX project using the usual "Add Reference". Then you can reference this file indirectly, by the name of the project, thus abstracting from the location and name of the DLL or EXE file. For example:
<Component Id="Assemblies" Guid="... some GUID ...">
<File Source="$(var.Product.TargetPath)" Id="entryPointAssembly"/>
<File Source="$(var.Help.TargetPath)" Id="Help.dll"/>
<File Source="$(var.Library.TargetPath)" Id="someLibrary.dll"/>
</Component>
Here, "Product", "Help" and "Library" are the names of the projects of the solution referenced by your WiX project. You should add project references using the tab "Projects" of the "Add Reference" window. Likewise, you can reference the output directory of the referenced projects and other attributes referenced here:
http://wixtoolset.org/documentation/manual/v3/votive/votive_project_references.html[
^].
—SA