Ok. I found a solution. Created a parameter(Name:ShowAll, DataType:Boolean, Visibility:Hidden, Default Values:False).
Added a textbox (Text:ExpandAll, Action:Go to report). Where users can click on, and all drilldowns expand.
And set the hidden visibility proporty of everything I want to expand to '= Not Parameters!ShowAll.Value'.
The problem Im facing now:
I have 2 tablix on a report, and the ExpandAll textbox action only work on 1 tablix.