Hkparts offers this service! The thread I’ll add below is old, but I spoke with them a couple months ago and they were still offering this. Zenith does it as well for a3f stocks, but not sure if they do so for 91s.
“We would be happy to provide that service for you. We offer this service to anyone that would like the service for $60 (with return shipping pre-paid) to professionally mill the extra slots and convert a 2 position stock to a 4 position stock. This applies to the MP5/94, HK93/33 and HK91/G3 stocks as well. We would be happy to provide this service to any or all of them. We can even put the cuts at your designated location if you have a preference or use the factory locations by default. Give us a call or send us an e-mail and we can get you setup. Turnaround is about 2 weeks start to finish :)