The units are defined by the type of channels.
All the values are saved in plain units, with no modifiers.
For example, values in channels with type "EEG" are interpreted as Volts.
To have milliVolts instead: multiply all the values by 10^-3 (or divide by 1000).
I have input signals from function generator that I obtain in milivolts as shown in the figure attached. How can I give this as the input data to the simulate generic signal?
If you have the signals already, you don't need to simulate them.
You can create a file with the simulation process, and then replace the fields Values and Time with your own signal.
For learning how to manipulated the Brainstorm data structures, refer to the Scripting tutorial.
https://neuroimage.usc.edu/brainstorm/Tutorials/Scripting#Export.2FImport_with_the_database_explorer