You don't. The plug (cap) has a different configuration between the two voltages. This is to prevent plugging a device of one voltage into a supply of a different voltage.
you get an adapter
You will burn up your appliance!!!!!
If I understand your question, Can you run a 110 appliance off of a 220 volt outlet? You could by only using one leg of the 220. You may also consider changing the outlet. Put in a regular 110 outlet and capping off one leg of the 220 in the wall box. You still have a neutral and a ground to work with. I read this as the questioner doesn't want to modify the outlet. Check the voltage ratings on the device. Many electronic devices nowadays can havdle 120V or 240V as it is cheaper to manufacture one powersupply and sell it everywhere. If your device is rated for 240V, you can put a 240V plug on it. If it only says 120V, no dice. 240V will burn it up.
Not unless the appliance is rated to be used with the two different power supplies. The US uses 120v, 60 Hz if it plugs into a receptacle, and possibly 240v, 60Hz if it is hard wired. The UK uses 240v, 50Hz whether it plugs in or is hard wired. These two power systems are NOT compatible. You may use a US appliance in the UK, and vice versa, only if the appliance is dual rated, i.e. the appliance says 120v/60Hz AND 240v/50Hz on the nameplate. If the appliance only specifies one power supply, it can only be used on that power supply. If this is the case, you may be able to use a travel adapter to operate the appliance.
Off hand no but your explanations are not clear as to who is doing what to whom,,
You don't unless it shows a dual rating on the appliance.
You cannot use 2-120v outlets to power a 240v dryer. You can convert a 240v dryer outlet to power 2-120v outlets if they are supplied with a neutral. This requires a competent electrician. Do not do this yourself.
Yes, but you will only get one quarter of the wattage from the heater.