I don't see any reason why a needle exchange pharmacy would refuse to dispose of them for you. The point of it (DAD-JOKE ALERT), is enabling the safe-disposal of needles and they don't ask what returned needles have been used for. I'm sure if you explained to the pharmacist, they'd be happy to put your bin inside the needle exchange bin. And if they won't let you then they're clearly a dick.
problem is budgets, the disposal of sharps from needle exchange comes from needle exchange budget, if we took in every sharps bin it would put financial strain on needle exchange. I will always except one as a ''one off'', but never agree to accept it as a regular part of our service.