Hi. Sorry for the absurdly late response.
We encourage everyone to build extensions to Feng Office.
In terms of licensing, you have two options:
1. Release it to the public under the GPL.
2. Talk to our Business Development people for support on how to sell it and distribute it.
For licensing an extension of a GPL product under different terms you need authorization from the authors of the work. It is not our preference to have closed source products, but we understand there are business reasons that could encourage it.