I would not recommend expiring any updates due to this. It shouldn't affect updates that are already published, and it should actually be able to see third party updates that are already published.
I am doubtful it will work with the setup you described based on our requirements, but make sure to check the full guide, here:
The new ad-in will recognize the updates previously published with SCUP.
You do not need to expire the updates you previously published with SCUP.
You don't have to do anything special with packages for the new add-in patches, The previously published updates will continue work.
To answer your question regarding if you can install the add-on I need to know the version and OS version of your WSUS server. To publish the version of your remote tools has to be the same version as your WSUS server. SCUP had the same issue. If your WSUS server is also on the 2008 R2 box then your using WSUS 3.0 SP2. Your remote tools on Windows 8.1 will not match. Let us know the details of your WSUS server.
My SCCM server is on Windows 2008 R2 SP1 and WSUS 3.0 SP2 is also on the same server.
My client computer is Windows 8.1. Remote tools are from Windows Server 2012.
Can I install WSUS console from WSUS SP2 installation on Windows 8.1?
Or I need to stay on SCCM Server's SCCM console and install there SPatch? In this scenario all operations must be done from server console. This is what I do now for SCUP.
You can not install WSUS 3.0 SP2 client tools on 8.1. If you install Shavlik Patch on the SCCM server it will work.