Hello, you have to follow poly curve tool bar for simulating conical shell foundation or otherwise, you prefer conical shell foundation in auto cadd and import the drawn foundation to PLAXIS..I suggest you to read PLAXIS scientific and reference manual for learning how to import the drawn geometry to PLAXIS..ok all the best..
you can do this easily by first drawing polycurves at different elevations and after that select the two polycurves and right click and select the loft polycurves.