As far as I know, this is not possible. DAQ Assistant is meant to be used when programming the application, not during execution.
To achieve your objective, you would have to do one of two things:
1 - Configure the channel directly in your application using "DAQmx Create Virtual Channel.vi" and have different cases for different instances (AI Voltage, AI Current, AI Temp TC, etc.) while letting the user select the instance through the GUI
2 - On the machine where the application is running, have the operator change the Task or Global Virtual Channel in MAX (Measurement and Automation Explorer) and use those in your application