I do not believe that you can do this in the client side SDK.
One option is to create a UCMA application that will respond to an incoming conversation by going to the conference and returning the URI to the user.
Then the user client can start a conversation with your application and send IM. Then the conversation will grow into a conference, and the URI will return from the application as IM. Your user client can handle URIs as they like.
The UCMA SDK contains a sufficient code example to help you execute quickly and quickly.
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