Yes, you can. In the 2nd screen (I think) of the Import External Data wizard there is an option to select 'specify a query' (what will skip most GUI screens). For instance if you have columns a, b and c on source and want to import a+b to x on target and c to y on target you specify
SELECT a+b as x, c as y
This is a feature that we never documented in detail (what we should). You can ask for specific help for your case here or in a ticket if you need. But please try it with a few very simple examples yourself too.