As mentioned above, its possible but its rude, disrespectful and can be dangerous. When you cast your circle, you could ask a specific God or Goddess to join you. Summoning is saying 'get over here now'. Build up a relationship with the deity in question. Set up an altar, prayer, offerings, are just a few ways you can grow closer with and connect with the God and/or Goddess.
If I were you I would research the deity well before calling on them. If you summon a god of war [for example] and tell them to attack your enemy, you could get hurt instead because this deity might have a temper, or expect a payment which you don't have. It's wiser to connect with them than call on forces you know little about.