Thought I already had a Python script to do this, but not, so I hacked one together quickly for those that want (esp. Mac Rhino people). If curves are not preselected, sticky option to join all into one closed curve or leave separate. Should work with any set of two curves (lines or not), and I found it amusing to add code to make it work if only one end can be connected (such as selecting two legs of an open triangle). Will fail if both line segments cannot be made - such as if the two selected curves actually touch at both start/end points.
Connect2CrvsWLines.py (3.4 KB)