Expression encoder SDK can accept sources from a LiveDeviceSource or from a LiveFileSource, but both can't support mms streams. The question is, is there a way to include a live mms stream in an encoding LiveJob?
And then, possibily a way to mix them and stream it?
I don't know if you can out of the box, but one possibility is to send the stream to a virtual video device first, which can then be encoded from a LiveDeviceSource.
Just with a quick google, http://www.soundmorning.com seems to have a piece of software called VirtualCamera that may do this for you. I have not used this particular software, but have done something very similar with an Axis Network Camera to Virtual Webcam driver.