How an agnostic might be said to have a relationship with God

You can be said to have a relationship with God if you have an internal dialogue with Him. At the very least it is asking Him for stuff in the form of prayer.  

This relationship which is in essence a relationship of bargaining would be improved if we remember to make prayers of thanks when our prayers are answered.  

What I will say about my relationship with God is that my prayers are drafted like contractual terms.

Even if God does not exist, the act of asking for specific things and going through why you ought to have your prayers answered concentrates the mind.