It is a little different than BricsCAD... BricsCAD convert always to LWPOLYLINE but correct in 3D... My lisp converts to 3DPOLY if UCS is not aligned with 2 lines, so I don't quite know, I thought PEDIT Multiple Join would be suficcient, but somehow it won't work on my PC CAD 2018... So it converts to 3DPOLY, but as long as it don't disrupt 3D info like JOIN did, I think it's OK...
I autoload it in my acaddoc.lsp, so that now if I want to code for (command "_.JOIN" ss ""), I would firstly (sssetfirst nil ss) and then (c:joinlsp)...