I understand you @oliver
You can highlight the time section and put your values in it. This will move the section line and you can cut (⌘X) and paste (⌘V) your new keyframe at that line. Not exactly what you are asking but a workaround until your request is perhaps added.
Also, in case you were unaware, you can shift the keyframe (1 frame and 30 frames) by using the key combination (right arrow → or ⇧→ respectively)
Again not what you are asking but it may help. Good suggestion.