Hi PhilthyCollins,
No, it is not currently possible to turn on the "Enable Optional eBay Gallery Images" flag as part of the CSV upload process so, if you would like to display more than one Gallery image in listings you are creating in this manner, you would need to add that option to each listing individually once they have been imported to your Saved Listings page.
I will pass along your comments in this regard to our Product Management team as a suggestion for future consideration though.
-Mike