Well, the easy way is
adjugate(a) = det(a)a-1
but this only works if a is invertible. If not, I guess you could just manually compute each cofactor:
Define adjugate(a)=
Func
Local adj,c,i,j,k,n
n:=rowDim(a)
adj:=newMat(n,n)
For i,1,n
For j,1,n
c:=a
For k,1,n
c[j,k]:=0
c[k,i]:=0
EndFor
c[j,i]:=1
adj[i,j]:=det(c)
EndFor
EndFor
Return adj
EndFunc