Have a look at this.
You will find a symbol which I defined for a stalagmite boss which I guess is what you are talking about.
def p_u_boss (expr pos,theta,sc,al)=
T:=identity aligned al rotated theta scaled sc shifted pos;
pickup PenD;
p := (0.08u,0.25u)..(0,0.29u)..(-0.08u,0.25u);
q := (0.16u,0.5u)..(0u,0.58u)..(-0.16u,0.5u);
for i=0 upto 9:
thdraw p rotated 36i;
thdraw q rotated 36i;
endfor
p := fullcircle scaled 0.15u;
thdraw p;
enddef;
To use a user defined symbol click on type U in the points drop down and then after the U, type in a colon followed by the name of your symbol so, in the case of the above "U:boss"