Hi,
putting a modifier on top of a Forest Pack object will force it to collapse to an editable mesh object and remove its instancing features. Unless you are only scattering very low poly objects it's not recommended.
There might be other ways to achieve your aims though. We only offer limited support for the lite version, but if you have a pro license please feel free to post in the pro forum with what you're trying to achieve and we'll do our best to help out.
Paul