In Visual Studio (at least 2017) you can add a WPF Window to a WinForms project by slighty modifying the .csproj file.
Just add
<ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
To the end of the first
<PropertyGroup>
tag.
It ends up looking something like this:
<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0">
<PropertyGroup>
<ProjectType>Local</ProjectType>
.
.
.
<ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
</PropertyGroup>
.
.
.
As always, feel free to comment, or ask.