You can add null-realtionships the same way as regular relationships. Open your target ItemType and click on the 'Relationship Types' tab. When you now use "Pick Related", you would insert a regular relationship that links to another ItemType. For a Null-relationship, change the selector to "No Related" and add a new Relationship Item. For Null-Relationships, you just have to assign a Relationship Name (=name of the Relationship ItemType) and a Tab label. If you take a look at the Part ItemType, you can see a good example how the two relationship types are used in practice. Null relationships are indicated by the orange 'plus' on the left side.