I don't think AsmOne (or AsmPro) has such a mechanism. You can emulate it to some degree by using a recursive macro, like this:
M macro
; Do something with \1
if "\2"<>""
M \2,\3,\4,\5,\6,\7,\8,\9,\10,\11,\12,\13,\14
,\15,\16,\17,\18
endc
(where the arguments should be on the same line, but something breaks it up).
As you can see, any argument not given simply becomes empty. I seem to recall that the maximum number of macro arguments is 19.
