In C++ I can do something like this to automatically sum custom class/struct (datum in DM) instead of calling function:
inline b2Mat22 operator + (const b2Mat22& A, const b2Mat22& B)
return b2Mat22(A.col1 + B.col1, A.col2 + B.col2);

Can something similar be done in DM, or I'll have to make function called Sum() and call it manually?
DM doesn't support overloading. You'll have to use a function.