@LeAnts if you are moving it into a combined network you can simply split the combined network into its components and then join together the bits you want.
For example:
Existing network A consists of Switch Network A, Appliance Network A and Wireless Network A, but you want to get the MX that is in Appliance Network B
Split network A
Combine Switch Network A, Appliance Network B and Wireless Network A into new Network A
TaDa!