To transform a classical library project to a WPF library project (in order to add UserControls, Windows, ResourcesDictionaries, etc.) you can add the following XML in the .csproj file in the first PropertyGroup Node :
<ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
Full example :
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
<PropertyGroup>
<Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
<Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
<ProjectGuid>{50E8AAEA-5CED-46BE-AC9A-B7EEF9F5D4C9}</ProjectGuid>
<OutputType>Library</OutputType>
<AppDesignerFolder>Properties</AppDesignerFolder>
<RootNamespace>WpfApplication2</RootNamespace>
<AssemblyName>WpfApplication2</AssemblyName>
<TargetFrameworkVersion>v4.6.1</TargetFrameworkVersion>
<FileAlignment>512</FileAlignment>
<ProjectTypeGuids>{60dc8134-eba5-43b8-bcc9-bb4bc16c2548};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
<WarningLevel>4</WarningLevel>
<AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
<TargetFrameworkProfile />
</PropertyGroup>
<!-- ... -->