Suppose someone offers a bounty and i want to increase that bounty - is there a way to do that? If not is it possible to set it up that others can increase a bounty.
It's not possible at the moment, no: https://judaism.stackexchange.com/privileges/set-bounties says that to set a bounty on a question, it can't already have one. As for the feature request, it's been asked before, both as an idea of adding to someone else's bounty and as an idea of having two users post simultaneous independent bounties on a question. I don't see that any real answer was offered to the latter question, though. (When you read the adding-to-another's-bounty question, bear in mind that it was written during a time that one could offer a bounty only on his own question.)