No, you can not. Visual Dialplan reads (but does not change) the Asterisk server configuration data. You can use all the configuration objects read from the Asterisk server for the purpose of the dialplan development but you can not create new configuration object.
However, Visual Dialplan supports Elastix, PBX In A Flash, trixbox and all other major Asterisk GUIs.
You are welcome to use these GUI tools to create extensions, voicemail boxes and other Asterisk server configuration objects that you may need, and then use it in Visual Dialplan.