Yes, you can use flea shampoo with Frontline but not until after the application has completely dried on the cat or dog, which usually takes about 24 hours.
After that time, you can get your dog wet and give him a good bath with your favorite flea shampoo, and it will not reduce the effectiveness or flea protection that he gets from Frontline flea medication.
In fact, you can combine Frontline with most other types of flea control products without any conflict.
Also, be aware that you do not want to give the application of Frontline after a bath or when the animal is wet. The cat or dog needs to be completely dry at the time of application and for about 24 hours thereafter.