Hello Adrian:
You can have several sources active at once. As Francois mentioned, you can edit all the source time series in the source space of your model. In brainstorm, we essentially use full-cortical source surfaces. Hence you can edit each every time course at each and every cortical location to simulate pretty sophisticated source models. Please let us know if the directions provided by Francois are not clear enough. We’ll have a dedicated web page for that eventually, so feedback is appreciated.
As for your second question, if export to matlab is not featured in your version, make sure you are running brainstorm with Matlab, not the compiled version (which does not require matlab).
Cheers,