We also converted existing libraries to portable libraries and it works fine. You have to modify the project file. Replace the following line:
<Import Project="..." />
with
<Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" />
Add following line inside a PropertyGroup tag
<ProjectTypeGuids>{786C830F-07A1-408B-BD7F-6EE04809D6DB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
And remove following lines from the AssemblyInfo.cs file
[assembly: ComVisible(false)]
[assembly: Guid("...")]
After that, Visual Studio should show you the Tab page “Library” in the project Property page and you can change the target frameworks for the portable library.