If you are a LE entity, the agency is eligible to purchase a brand new HK 53 from HK, or one of their many LE dealers around the US.
While it is true that you could have a Class II manfuacturer convert a 93 into a 53, I can't imagine that you would find that cost effective, or that the legal folks from your agency would find that the potential liability from an after market conversion would be less than that offered by the original manufacturer. Of course if you were talking about a 91 and a 51, it would be understandable as the 51 isn't a "real" factory gun.
You can easily get a 53 from HK, I'd suggest you go this route. I'd be surprised if Stan or Terry would accept this project anyway...