You can not currently upload Sustaining gifts via Common Ground or LCRM.  Instead they must be added on the Luminate side by doing the following:
  1. Find the constituent profile of the person making the gift.
  2. Select the "Transactions" tab
  3. Under Gift Entry, select the correct form and hit "go"
  4. Fill out the donor info and Credit Card info, and save/finish.

The gift can now be viewed by the User in their service center, or you can edit it from the back end as needed.