Embodiments of the present disclosure are directed to apparatuses, systems, and methods for merging a balloon catheter onto a locked guidewire. In one implementation, a balloon catheter may include an inflatable balloon affixed thereto and a slit extending from a distal end of a guidewire lumen to a position proximal of the balloon. The slit may be widened by a working member of an adapter to allow passage of the locked guidewire into the guidewire lumen of the balloon catheter. The balloon catheter may be merged onto the guidewire via the slit and delivered to the desired treatment device without requiring the guidewire to be unlocked. Advantageously, access to at least one desired treatment site may be maintained with the guidewire during merging of the balloon catheter.