You could do this, but it would have a lot of limitations. First, you'd need a phone attached to the Legend to initiate the conference bridge. Second, you are limited to a total of 5 parties (including the user on the Legend phone.) 2 of those parties can be connected via outside lines, and 2 can be inside lines. So, if you want a "resource" that your Definity can talk to, you'd be using single line station ports on the Legend. Thus, your practical limit is 2 parties. Third, audio quality drops when Legend conferences analog lines.
Doesn't Definity allow placing conference calls?
If your need is infrequent, consider renting conference bridge service; prices are below 10c per minute per line now, and calls can be web-managed, and billed to the department/project that demands them.