In RC, you can do the following:
foreach i [get_attr instances [find / -subdesign add_sub]] {
set_attr unresolved true $i
}
write_hdl > top.vg
The above script finds all instances of add_sub, and sets an "unresolved" attribute on each instance. This TCL could, of course, be simplified if there is only one instance of add_sub.
write_hdl will now omit the add_sub module from the netlist.