Custom field & sub custom field

[ahecht]

I created a new dropdown custom field (Module)
Can I create a Sub Module field for each module type ?

module Sub Modules
m(1) (1-1, 1-2 ,1-3 … )
m(2) (2-1, 2-2 ,2-3 … )
.
.
m(n) (n-1, n-2 ,n-3 … )