As I know, default Magento admin import method does not allow to import custom options in bulk. So you can use any 3-party solution.
I can offer you to use Store Manager for Magento, since there is fully functional 2-weeks version available for download -
So you can do this process without having to pay, as free version will be active for 14 days and I guess during that period you will be able to cope with the task (in case you do not need to do that regularly).
Also read this article on how to import custom options via Store Manager -