In that case, here's what you can do. This will give the computer the
appearance of using a
pre-placed Canal, which seems good enough for your purposes.
Firstly, you place a Nydus Canal wherever you want it to be. Then, make a location around the Nydus Canal, make it a bit large. At least this large:
I'll refer to it as "DOOMTUNNEL" from now on. Then, make another location at wherever you want the Zerg to attack; I'll refer to that location as "OHSHIT" from now on.
Then, create a switch. I trust that you know how these work. I'll refer to that switch as "Rush" from now on.
Then, you can create the following Triggers.
CONDITIONS:
[Whatever you want to actually start the attack, whether that is the player building a few units, a certain amount of time elapsing, a switch being set or cleared,or bringing a unit to a certain location]
ACTIONS:
Set switch "Rush"
Then
CONDITIONS:
"Rush" is set. [This trigger is just labelled "Switch" in the editor]
(AI Player) brings at least 1 buildings to "DOOMTUNNEL"
ACTIONS:
Wait for [Some amount of time, if you want there to be a time delay between waves of Zerg. If this action is absent, Zerg will just pour out nonstop with absolutely no delay]
Create [Desired Number] [Desired Unit] at "DOOMTUNNEL" for (AI Player) [You can use this action multiple times if you want there to be a few different units.]
Play sound\Misc\IntoNydus.wav
Issue Order to all men owned by (AI Player) at "DOOMTUNNEL" attack to "OHSHIT"
Preserve Trigger [This will cause the Trigger to execute continuously as long as the conditions are met. If the "Preserve Trigger" action is absent, then the Trigger will only fire once and never fire again].
This trigger will cause Zerg to appear at a Nydus Canal and attack a certain area once something sets it off and for as long as the Nydus Canal is alive.
Now, you need to import sound files to your map to actually use the play trigger. To do this, you go to Scenario>>>Sound Editor in your map editor. Then, look in the "Available game sound files (Virtual Sounds) box that appears. It should be the bottom one.
Find the IntoNydus file, click it once so that it is highlighted in blue, then click the "Add!" button. Now you can play it on your map.