Import photos from external disk into iCloud Photo Library via MacBook Air with not enough HD space
First, you will have to update to Photos app on OS X, as iPhoto is not compatible with iCloud Photo Library.
Activate iCloud Photo Library and subscribe a storage plan that accommodates your pictures and anything else you have there (iOS Backups, for instance). Select the "Optimize Mac Storage" on the Photos app Preferences > iCloud pane. 

Then import batches of 10 GB or so of your pictures to the empty Photos app library; given you will have only 5 GB left I assume it will start optimizing right away. When you regain some space, you can add another batch.