When working with locked elements, the ability to add connectors depends on the locked status of the source and target elements. The rules are:
|·||Source unlocked, target unlocked: any kind of connector can be added|
|·||Source unlocked, target locked: allowed, except for composition connectors|
|·||Source locked, target unlocked: prohibited, except for composition connectors|
|·||Source locked, target locked: prohibited for all connectors.|
That is, a connector can be added if its source is unlocked, regardless of the locking state of the destination (think of it as modifying what the source can see). The exception is composition connectors, where the target (i.e. parent) must be unlocked (think of it as modifying the parent by adding children).