I don't think there really is a creative way to do it, and I had more than a few ideas flow through my head that I discounted....
I have had one similar situation to this. My main site already had X2400-2499 (funny the numbers are the same!) and we had another site that decided to be a lone ranger and hire a local company to replace their phone system before we had a chance to integrate them into our network. Of course no one bothered to consult me regarding the numbering plan either, and they ended up getting DID with 2400 - 2459 for the extensions.
Now it comes time to integrate them into our network and we have this big number clash going on. Fortunately the numbers were in a different NXX than my main site, and I came up with a really klugey solution that lasted about 5 years until a combination of two things happened. The first was that I successfully petitioned the Public Service commission to give me 60% of a K-block (10,000 numbers) under a new NXX in my town and I consolidated my entire dial plan into one NXX (previously I had 682, 683, 684, 686, 652 and maybe one or two others. Now everything is 320-xxxx), and the second thing is the site moved, and we had to change all of the letterhead, brochures, yadda yadda anyway, and advertise the move, so I advertised all new phone numbers for the site. I hung onto the old ones for 6 months and then had them shut off with a forwarding announcement on them).
My klugey solution was that my whole network is standardized on 4-digit dialing. For that particular site, when we added them to the network we notified everyone their numbers would be 44xx, with the last 3 digits staying the same. So the old number for the site was 2400 to the outside world, and for internal calls it became 4400. So for external calls the customers dialed NXX-2400-2459, and inside calls across CorNet dialed 4400-4459.
Now I know some of you are rolling your eyes back up into your heads, but my people had never called this site before internally so they got used to it really fast and we had no problems.
How did I deal with this from the outside?? Well, all of my sites receive the last 4-digits of the called number from the phone company and that is how the calls are routed to each extension on a 1:1 basis. In the case of this one site I asked the phone company to just send me the last 3-digits, and I told the phone system to automatically append a "4" to anything that came in from outside. So the customers dialed NXX-2400, and I basically stripped off the 2 and replaced it with a 4 - the customers had no clue.
Then after we migrated to the new NXX it was really simple to make the change. I designated 320-4400 thru 4499 to go to that site, still only sending 3 digits (400 - 499). When we moved we advertised that we were moving and our numbers were changing, and for every 24xx number we advertized the corresponding 44xx number now. The phone company gave us a 6-month transition period before I had to give the old numbers back, so the customers (for all our sites) basically had 6 months to get used to the old numbers while they could still get through on the old ones.
Basically I completely overlaid my dial plan. It did not matter if the old number was 682-2400, 683-3200, 684-5500 - now the new numbers were all 320-2400, 320-3200, and 320-5500, but since the phone company was only sending us the last 4 digits it made no difference what the first 3 were (except in the case of the weird site where they only send 3, but the same logic applies). So I effectively changed out and consolidated the dial plan of our entire organization and didn't have to type so much as one command into a single PBX to do it! The only thing I needed to do was a minor thing at the goofy site, and I had to do that anyway.
So this whole long winded story is to suggest that maybe you can do something similar. Force that site into a 4-digit dial plan. Append a digit at the beginning of those 240 extensions. Your people will get used to it really fast because they have the exact same extension number and you are just adding a single digit in front of it. If people are used to dialing 240, now they dial 3400 or something. If that site has DID and you are running those 3-digit extensions, more than likely the phone company is only sending you 3-digits anyway, so you just go into your DIDCR tables and tell the system to add your first digit in and you are all set to go. The outside people don't need to know anything is different.
Just a suggestion.