To be more specific you cannot modify a domain that is used for Asset Types without disabling the network topology first., you can learn more about what network management tasks require you to disable the topology in the network management tasks in the online help.
Taking this a step further, even once you've added an asset type to a domain you will still need to add connectivity rules, network categories, etc to this asset type. All of these tasks require the topology to be disabled, so I recommend you figure all of this out ahead of time in a staging environment or file geodatabase so you only have to disable the topology and affect your production users once.