Zaheer is not allowed to sign as an overseas player as he does not possess a working visa but wants match practise to boost his hopes of a place in the India squad for the Asia Cup.
He played his first match against Essex in the National League after Pakistani spinner Saqlain Mushtaq was ruled out with a knee injury.
"The arrangement suits both parties," a Surrey spokesman told the BBC.
The Indian has been dogged by leg injuries in recent times and has not played since limping off the field in the first Test against Pakistan in Multan in March.
"I would think he'll definitely play in the next championship game and maybe the one after that, but we'll just have to see," the Surrey spokesman added.
"Zaheer has been training with us for the past week and wanted to sign as an overseas player.
"But when he couldn't he decided to play anyway in an amateur capacity. He just wants to prove to the Indian selectors that he is fit and playing well."
Saqlain is due to undergo an MRI scan at the end of the month before determining when he will return to action.