Cal,
To add Legacy to your DCS layout you need a Legacy 990 Unit, plug the 990 Base unit directly into your properly grounded house plug outlet for best Legacy running results. Run the Track line from your Legacy Base unit directly to the oppsite outside Track Rail, that your DCS Black Channel is connected too. Then split the Legacy Track line coming directly off the Legacy Base Unit, and run the Legacy base unit Track line, to each of the Black outboard DCS Channels on your TIU, setting up a Dual Legacy signal on both out side rails of your complete layout, this sets up a very strong Legacy signal thru out your complete layout. In reality you do not need a Legacy 993 unit at all, if your Legacy 990 unit is functioning properly.
You might want to Check out the Lionel Legacy Educational Videos on the Net, and Barry does a great job of technical direction writing in his DCS O Gauge companion also.
Have Fun!
PCRR/Dave