Well, yes, it is possible, but I wouldn't advice.
Boiling water changes water hardness.
Temporary hardness is mostly destroyed by mere boiling of water, when bicarbonates are decomposed, will produce insoluble carbonates or hydroxides, which are deposited as a crust at the bottom of vessel. source, related Wikipedia link
Boiling water can increase its pH:
boiling the water would also have removed any dissolved CO2. Dissolved CO2 will form a small amount of carbonic acid, which can significantly lower the pH of very soft water. The fresh water would then have a lower pH than boiled water. source
What about temperature?
If you pour hot water directly to the tank, you risk that a fish will swim just under the hot water. Maybe some dirt will raise up because of water moving the bottom, which will cause fish to think it is food or something else worth examining. Even if this water is getting cooler relatively fast, it is still hot and can harm the fish, if it will not manage to escape. Maybe consider inserting another tank (made of glass if you are able not to destroy this with temperature differences, or metal) in the aquarium, so there is no risk that any fish touches the hot water, but this take space of your tank.
You can calculate what amount of waters of what temperature you should mix to have the temperature of the sum, because it would be the weighted mean. So the final temperature
T will be
T = (Tt*Vt + Th*Vh)/(Vt+Vh) where
Tt is the temperature in the tank of volume
Th is the temperature of hot water of volume
For example, you have a
100 l of water in your tank of
23 deg C and you add
1 l of water of
98 deg C this would lead to
101 l of ca.
23.7 deg C. This would be true if the waters mix fully, however they will never do. The warmer part will get cooler because of air and glasses. In fact, you will never reach stable temperature of
23.7 deg C in the whole aquarium.
I would agree with rib.usa that it is in fact not worth effort. The water will soon cool down because the air has lower temperature and you will need to repeat the process, probably just after an hour. But your tank does not have an infinite volume, so you need to take some water out. Every time you risk a stress for fish and you change water parameters, as shown above.
I would invest and buy a heater. They have thermostats and keeping right temperature is no effort from you. I think you can buy one for about 30-50 USD. If you are afraid of cost - I have 100 W heater which works about one third time a day, let's say 8 hours. This makes about 0,8 kWh a day, or 24 kWh a month. I don't know what is the price of energy in your country, in mine it is about 15-20 US cents. So the total monthly cost is about 5 USD. You can calculate yourself how much money would you spend to warm water by other means and how much time you are going to waste for preparing this water (time is money, too, remember).