There are two ways to add a league to an existing chain, however you must know the chain ID and password before continuing with either option.
Option 1 (league already exists): If the league you wish to add to the chain has already been created and is filling up with teams, or is full and play has already started, you can still add it to a chain at any time. However, you must be the founder of the league to add it to a chain. If you are the founder, so to the League Office. There is a blue box at the top of the right column with the theme rules and description in it. At the bottom of this box, there will be a "Chain" option. Click "Chain" and enter the chain ID and password. Your league will be instantly added to the chain.
Option 2 (league has not yet been created): To start a new league that will be part of an existing chain, you must first have purchased a full season hockey team. From the Team Center, find your new team in the "Drafting Teams" section and click the "DRAFT CENTRE >" link. The next screen you'll see is the "Drafting : League Options" screen. If you do not, click "Join A New League" at the top of the Draft Centre. On the League Options screen, click the "More League Options >" link. The page expands to show more options. Now choose "Create A New Season For An Existing Progressive League Chain" and continue. Once you have drafted your team, entered a team name and selected a home arena, you'll be prompted for the chain ID and password. After you enter this information, you'll continue on to choose the theme options. Once complete, your new team will be in a new league that is part of the chain you specified.
For more information on Progressive Chains, click here.
For information on how to start a Progressive Chain, click here.