It is possible to add Dispatches to the Upcoming Tab from outside of the Upcoming Tab.
The key to doing this is to make sure that the Dispatches are scheduled, that is, they are not set as "Immediate."
Here's what that looks like on the Create Dispatch screen:
If you set the Dispatch Type to Immediate, it will not appear on the Upcoming Tab.
This same logic applies to bulk uploading Dispatches; if a date and time are entered in the Scheduled At column and a time is specified in the Duration column, the Dispatch will appear on the Upcoming Tab.