Association/navigation for non-key fields of entities

Dear Experts,

I am building a service for Sales order. My structure as follows -

Entities:

1. VBAK

Key : vbeln

non-key : kunnr

-- there is no matnr

2. VBAP

Key : vbeln

non-key: matnr

-- there is no kunnr

3. KNA1

Key : kunnr

4. MARA

Key : matnr

We have association exist already for VBAP and VBAK.

now my requirement is to get VBAK and its corresponding KNA1. Similar way VBAP --> MARA

How can we do the association/navigation using non-key fields?

Thank you,

Murali