Hi tualatin,

I assume that this is your current diagram.



















1. Add a Class A Lifeline and create an initialize message. *When you create it, draw it toward the sendMessage's source activation.
















The initialize message is created. (You don't have to change the source of sendMessage.)












2. Now what I would do is, move the target activation of composeMessage (*The activation in green) onto Class A temporary.





















The green activation is on Class A.



















3. Move the origin of composeMessage to the target activation of initialize message(*The activation in yellow)





















Now the activations on Class B got all in one.



















4. At last, put the green activation back onto Class B and its done.




















Hope this is what you wanted to do.

Best regards,
Satomi