I was wondering if it's possible/advisable to give a flowering plant or any plant for that matter, sugar (or something similar) in addition to plant food?
Well you could in solution, but I wouldn't advise it. If your plants are growing well anyway, sugar might just kill them. Sugar molecules don't pass directly into the plant, so will build up in the soil in the pot, and cause problems later if you always add sugar. Sugar also directly feeds microbes in the soil - some of these may be beneficial, but some may not be, and sugar doesn't discriminate. Sugar solution can also cause reverse osmosis, meaning water or fluid, rather than being taken up and then utilized by the plant, is instead expelled, causing the plant to die.
I'd recommend you fertilise with a standard, proprietary fertiliser at the recommended rate on the product and forget about adding sugar.
Probably not a great idea - more likely to attract insects than anything else.
This abstract suggests that it will decrease plant growth.
Most plants (with a few exceptions) are happiest if you give them a nice rich compost, IME. Some are overly sensitive and need it very well aged, and in moderation, others grow happily on the compost pile itself, long before it's "finished" and with no trace of moderation.