Originally Posted by Entropy
You can currently do that for all the rewards that they DO currently have available(such as the digital game guide). I asked them that.
PM reply from Numantian Games:
'Yes, you can do that.
Pledge 5$ [for example, for digital game guide] more without choosing a reward and send us an email to firstname.lastname@example.org with something like "I want to upgrade my reward" and we will register your new reward.'
Also, have you looked at the $5 tier? It's an "add-on ONLY" tier designed just for indiegogo people!