Yes, you can certainly add more campaigns and ad groups after you have submitted the account for review. The grants team does not expect to see a "whole" account, just enough campaigns and ad groups (3-4) to see that you understand AdWords and that you will be utilizing the grant effectively. After the account has been approved is when you will actually do much of the building out of the campaign.

Additionally, since "Active Management" is a requirement to keep the grant, rest assured, you will be building several more ad groups over the course managing the account. 

All the best,

Robert