I think that in this case ProcessBook add-in is useful
Either put common code in a .Net addin or in a common PDI file (where you reference the common PDI from all those you want to inherit the code from).
I did initiate a Community Project to address this very issue too, see the following link: http://vcampus.osisoft.com/forums/44.aspx
How about developing your own Class Library with some standard functionality, which you then add as a reference to your multiple PDI files' VBA project?
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