Can I simply add more fruit and sugar to a batch of must once I have drawn off the majority of the first fermentation?
You can always add new ingredients before, during or after fermentation.
However, be carefull to sanitize your equipement and wash the fruits carefully to avoid contamination of your must. I would also add the sugar to a small quantity of boiling water to kill bacteria and help dilute it, then add to the must once it has cooled sufficiently.
There is a youtube video where a guy just kept filling his conical fermenter with more wort for a whole year. I think he reported that as the yeast fermented more and more batches that it started producing different flavors. I think he said he got to 25 batches by the end of that year and that the last batch was the last he cared to do on that yeast because it was getting funky. Sorry I don't have a link, but it was either a catalyst or a fermentasaur if anyone is interested in taking a look.
Bottom line is yes you can, but if you want to do it right and over a long period of time or huge number of batches, you need to learn to rinse your yeast, otherwise you start introducing old junk into your wort if you are making beer, and even when making wine you are always introducing something that is potentially rotting and could affect it in the beginning before the alcohol starts preserving the mystery ingredient.
Doing it just a few times? No problem at all. It's a fun experiement, but yeast is really cheap. Don't do it to save a few pennies, do it because it's fun to try.