how can i add older disk games (C&C Generals) to my EA library

You can try redeeming the code on your EA App Library:

Go to Library > Redeem Code

If the code comes up as invalid it's very likely that the code is too old and cannot be redeemed. 

Thanks for the info.
I purchased the C&C collection thru amazon and redeemed the code in my library. Game played just fine but the next day the game was no longer in my library. Now the code indicates "already redeemed" upon attempted re- redeem. Can anyone fill me in on how to resolve?

If the code is showing already redeemed you will need to contact EA Help Support so they can check the code for you. If you managed to redeem the code once and play the game it must be registered in your account and hopefully, they will be able to assist with that.