Open steam.
Go to Library.
Right click on Half-Life, select Manage and select "Browse Local Game Files".
Half-Life folder will be opened.
In explorer, right click to address and copy it.
In your archive manager (7-Zip or WinRAR), select cs13 folder and select "Extract".
In the address section, paste the address of Half-Life folder, then select Extract.
After it finishes, close and restart Steam.
Counter-Strike 1.3 should appear in your library.
Use the images in "Library Images" to give it a unique look in your library.