Just keep you Re-Usable functions in external .vbs or .qfl file and associate these files with the test.
Just keep the test case scripts in your actions and call your methods inside cases wherever you want. External file functions are available for whole test not specific to actions.
We have the facility to call particular action with QTP's 'LoadAndRunAction' function. However there is no way that you can select functions even from selected Action. This is because the Action is one single entity for QTP, which can not be further granulated.