I assume that your question is actually about synths and not panoramas. (The answer is the same in both cases - no, you can't add a photo to a synth or a panorama that is already online - but I'd just like to be sure I'm understanding what you're asking.)
The reason that this is, is that both synths and panoramas are actually calculated on your own computer before they are uploaded. Adding photos to synths that are already online would either mean the synther (the Photosynth app that uploads your photos) downloading the data for that synth to combine with your photos to then upload back up to the website or, on the other hand, the Photosynth web site doing all the calculations itself (but that would be incredibly expensive for a small team that doesn't really make Microsoft any money). While neither of those things is technically impossible, they haven't been announced as planned, either.