The simplest way is just to try it 🙂
But yes, partial interfaces are allowed.
Valid locations for the partial modifier (with C# 3.0 spec references):
- Classes (10.1.2)
- Structs (11.1.2)
- Interfaces (13.1.2)
- Methods (C# 3.0+) (10.2.7; 10.6.8)
Section 10.2 of the spec contains most of the general details for partial types.
Invalid locations:
- Enums
- Delegates