I bought some sound packs for Sound Center that included guitars, ethnic, and strings on Steam a while back.
 
I now have Sonar through Cakewalk (Lifetime updates) but when I installed Sonar through their license management tool, I no longer have those sounds. They still appear in my Steam library but not in the My products page on Cakewalk.com. I bought these on a Steam sale with Music Creator before I switched to Sonar Platinum. How do I get these back? If I reload Steam and try to load them through that, will they see my installation and plug in accordingly?

I have no experience with Steam managed Cakewalk products and DSF expansion packs.
 
There is a way to sling DSF expansion packs around but it might be easier for you if Cakewalk adds the appropriate installers to your Cakewalk account.

I asked the same question recently and got nowhere.
http://forum.cakewalk.com/Steam-to-Platinum-m3148603-p2.aspx
 
I've been waiting weeks to hear back from support. I'll let you know if I find out anything.
 
Maybe reach out to Seth Kellogg (Cakewalk's Steam Channel Mgr) on the Steam forum?
http://steamcommunity.com/app/380080/discussions/
 

I will reach out to Seth. We're friends on Steam. Thanks for the advice.

Here is what Seth said on the Steam forums:
"There are 2 ways:

1. You'll need to install Cakewalk Sound Center application through Steam. Products purchased on Steam are only available to download through the Steam Client.

You can point SONAR's VST scanner to '[steamlibrary]\common\Cakewalk\Vstplugins\' location and they should work fine.

2. Alternatively, if you wanted to use the direct installer, you'll need to copy and backup the multisamples under: 
'[steamlibrary]\common\Cakewalk\Vstplugins\Cakewalk Sound Center' 

and the Programs under:
'[steamlibrary]\common\Cakewalk\Cakewalk Content\Cakewalk Sound Center\' from the Steam locations

Then copy them into the direct locations:
%appdata%\cakewalk\cakewalk sound center - for programs
Program Files(x86)\Cakewalk\Vstplugins - for multisamples"
 
The problem is that I wiped and reloaded my PC when I switched to the non-steam version. My OS was really wonky due to having so many versions of Cakewalk software installed. I posted this and will let you know how he replies.

Seth replied: "There should be a separate 'Cakewalk Sound Center' app in your Steam library, which doesn't require MC7/SONAR 2015/2016 to be installed."