Modifying building yields - bonus resources?

[FinnTitan]

Hi,

I hope there are some of those XML experts out there to answer my question. I want to add a new religious building (i.e. Hindu market). When built, it should add commerce (+2) to all fish bonus tiles inside that city's working area (city cross). Is this possible by changing file "CIV4BuildingInfos.xml"?

I found a line <BonusYieldModifiers> which (supposedly) changes percentages for production but I was looking for amounts (like in <YieldChanges/>) for bonus squares..

Hope you understand what I mean and thanks in advance!