Is this a 3-year extension? Did you arrive prior to your initial start date? As far as your question goes: your employer is petitioning for H1B but not for the visa stamping. Whether they can extend it or not has nothing to do with your current visa stamp's expiration date. Generally speaking if you have an approved I-140 then you should be good for extensions indefinitely.
If I am reading your post correctly, it is the visa that has a July expiration date, correct? If so, that has nothing to do with how long you can stay in the US without leaving or getting an extension. That decision will be made by the immigration officer at the port of entry. The POE officer cannot extend the visa, but he can give you a departure date that exceeds the validity of the visa. The visa is merely the last day the visa can be used to enter the US; has nothing to do with how long you can stay in the US.