Can you ask for a feeding tube even if you don't need it?

Not really: Hello, it would not be advisable as even when they are needed, feeding tubes should only be used when better options do not exist. You would open yourself up to the discomfort and risks associated with a foreign body without any need to do so. Definitely not advisable. More important is why you think you would benefit from one as your concerns may be adressed by a better method.