To add SignRequest to a custom object you should first add a lookup relation to your object on the SignRequest object.
Go to: Setup > Build > Create > Objects > SignRequest > Custom Fields & Relationships > New > Select a lookup relation to your object.
Below is a more step by step walkthrough of the process.
1. Select "Setup"
2. Select "Objects"
3. Select "SignRequest"
4. Select "Custom Fields & Relationships"
5. Select "New"
6. Select "Lookup Relationship" and click "next"
7. Choose your custom object and click "next"
8. Enter the field name, use the same name as the custom object name
9. You can now click "next" a couple times and save the lookup relation