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.
Most of this is rubbish, you know how they grow those massive pumpkins? They.make sure it's the only one then inject glucose directly into the stem. Plants can metabolism sucrose and fro rose and glucose. Use something natural like honey which is easier for them to absorb but it's mostly done to increase microbial activity to release more nutrients in organic soil. They will absorb sugar through their roots wether they want to or not but they mostly will simply excret the excess. But most plant tissue is cellulose, which is what? Chains of dextrose (the d isomers of glucose) anyone who thinks plants and animals are different are silly. Cells consume 2 things oxygen or sugars. This is true for all living organisms.
TLDR; vert crop dependant. Not advisable to a novice.