You cannot do this with most dogs. The dog that is on the leash is at a disadvantage in doggie land. If he starts to make any sounds of distress, every dog in the park will run to investigate which will put the leashed dog into even more distress and some may attack.
Almost every off leash dog park has a leash/un-leash vestibule. It is there for a reason. If you let your unfamiliar dog into the park free, the dogs will run up, investigate, sniff, greet, what have you. Most dogs will stand still until the inspection is over and go do their own thing. That is if they are free to run away if danger arises. If they are not free, they are very insecure and bad stuff can and does happen.
Personally, I scan the park for Pits and let my dogs in if there are none present. I know I will hear from the Pitbull folk but I keep my dogs safe how I see fit. A youngish pit is no problem but if I see an intact male over a year old, I keep my dogs away.
I like Pits personally but any moron who brings an intact male pitbull to a dog park shouldn’t have a dog and is too stupid to be intact himself.
I shudder to think what my reaction would have been, as I am always armed in the park................