You just want to limit the number of digits passed to the 3rd party PBX? Or are you trying to prevent users from using touch tones after they are connected to the PBX. If it's the first, yes, there are a couple ways to do this, I think.
The first and most powerful is a translation pattern. You can use a regular expression to add or remove digits to fir whatever your PBX requires. However, you really need to know regular expressions, or have time to play with it, to get it to work right.
Do you know how many digits you will be receiving from the PSTN? Do you want to strip the digits from the front or rear of the number? If it's from the front, you can use digit-strip within the dial-peer.