Yes, but it's a bit complicated.
USB in VoiceLive Rack is mainly intended for recording a complete live vocal performance, but with a Mac it is possible.
First, if you are using a Mac, they handle multiple audio devices through an "aggregate device". You'll need to go into Applications > Utilites and use Audio Midi Setup to create an aggregate device that contains your current I/O and the VoiceLive Rack. You can find a description for that here.
Second, the ability to insert the VoiceLive Rack into your effects chain will vary based on the DAW you're using. We've found that often the aggregate device renumbers all of your INs and OUTs without giving any chance to label them yourself. This means you'll have to figure out what numbers it has assigned to the VL Rack.
So all in all: yes, it's possible but requires that you put in the time to experiment and sort it out on your particular setup.