It seems that the numpad numbers and ‘.’ are not assignable with both Shift and Alt as modifiers while Num Lock is on.
Open up the Keys page of the preferences.
Make sure Num Lock is on.
Place your cursor in the new assignment box and hit Shift + Alt + Numpad [0-9, .].
It will register only as Alt + Numpad [0-9, .].
If you turn Num Lock off and then press Shift + Alt + Numpad [0-9, .], it will register correctly.
The same is also true for Shift + Control + Numpad [0-9, .].
Shift should register the same when recording combinations, independent of Num Lock, should it not?
PS: I also noticed that hitting the Clear button with no function selected in the browser will not clear the new assignment box until you change your selection in the browser.