Thank you for your quick reply!
The reason for the question was to merge the inventory data by exporting from a subnet and importing into a single transition. In that specific case, I want to create different Electricity Mix for specific cases. Thus, by copying the whole subnet, I think it will paste and calculate the whole module (and furthermore about 13 times the same material for each transition such as wind power, photovoltaic mix, hard coal etc.).
However, the copy&paste function through Module Gallery is a good way! Thank you so far.
As you said "not available yet", is it intended to implement such an import function with excel files?