I know of no way to do that. That is what DIDs are for. You SIP provider will not pass any extra digits that you append to the DID on to you.
Think about it. They make money selling DIDs. If they had a system to allow you to have a single DID and reach multiple people they would not have that revenue stream.
No it’s generally not possible, dialing a phone number is basically using the MSD (most significant digits) of E164 to complete a call to an answering service.
With the exception of some German and Austrian “exchanges” ( but they follow ISDN further than most) the butt stops there and the phone will ring, you will need to answer at that MSD conclusion by your system and so redirect downline at your whim.
I think you are right. Added extension gets swallowed by provider or at least it does not reach the PBX. Just checked the call logs and there is no entry when I dial +353-1-XXXXXXX100 from my mobile.
So I would possibly just buy 4 more DIDs and problem is sorted.
I just realized than when I said the “butt stops there” will confuse most. A butt is a butt-set, that phoneguys hang on their butts to diagnose phone shit, The “Buck stops here” is a term that generally means “it’s your shit from now on”. OK it’s an old joke, but one that I hope at least some old farts who know what they are talking about will appreciate.