Adding the off-diagonal mass to the Mass Matrix

[skh]

Hi,

I will appreciate you , if you could guide me how can I add off-diagonal mass to the Mass Matrix?

For example:
Current Mass Matrix

a 0 0
0 b 0
0 0 C

After Adding off-diagonal mass to the Mass Matrix :

a 0 0
0 b+d -d
0 -d C+d

Thank you in advance for your time and consideration.
best,