It took me a while to figure out the reason why i couldn't use files from pirated games to use in steam. At last i think i have found a solution. Problem is with directory name. Normally the pirates put any name to game directory as they wish. But steam only accesses the files only if the name matches to its directory info. For example when i pirated Middle Earth Shadow of Mordor, the directory name was the same "Middle Earth Shadow of Mordor" but in steam the directory name is ShadowOfMordor, for metal gear solid phantom pain the folder name is MGS_TPP etc. After buying from steam i saw that i couldnt use those files and i wasnt willing to download that 53gb of data again.
So i looked for the steam appid of the game. You can find it in the temp folder if you start downloading the game (steamapps\downloading\appid). Take a note of the digits. Search the appid in steamdb.info. there if you can see your appid is matching your game then look for additional information. There you can get your dir name.
but beware that this information is not always available. For example, i saw metal gear solid the phantom pain's dir name yesterday but today i couldn't find it. Now stop downloading the game, delete any file/ folder downloaded in downloading/appid folder. Put your game files under the directory name you have found in steamapps/common/yourgamename and restart steam. Steam will automatically verify the game files and may download some files which are changed. And voila!!! you have saved a lot of bandwidth.
This is not an official guide or anything. Just personal finding. If you have further information to add, you are welcome.
edit: now i can see metal gear solid the phantom pain's installdir in configuration tab.