Okay, here's a whole different way to approach this.
If, in fact, you can currently call one of your 5XX extensions by selecting the trunk group connecting to Asterix and dialing an extension you can actually make the Mitel phones call the Asterix phones by just dialing the extension.
As Tech said, you must renumber the CONFERENCE feature to something that doesn't start with 5.
Next create a PHANTOM EXTENSION 5. Manually forward that phantom (using 359 etc) to the Asterix trunk group 92005. When it asks for a destination number enter 5.
Now here's what happens when you dial an extension starting with 5: Call goes to phantom. Phantom, invoking its forward rule, grabs Asterix trunk, then dials 5 into the trunk (first digit of Asterix extension) and then caller continues dialing other digits. System is going to buffer the extra digits, so you don't need to worry that you're dialing too fast, caller just dials the entire extension. This will not work, however, from analog phones, maybe not SIP phones