I sent an e-mail to the Financial Aid Office last Friday and they have just answered me. They said that those who want to appeal their funding amount must fill a form by April 9. The problem is that they will respond after April 15. So, I would have to pay the enrollment fee to secure my place before knowing my final financial aid package, which sounds a bit odd.
In my particular case, I received some funding from SIPA and secured some more from external sources. All I need now to attend SIPA is a 50% increase in the scholarship amount I was originally offered. Does anyone have an opinion about how realistic is this request? I do not have any other award letter so far, as I have only applied to SIPA and HKS.