Its possible because Lrazier's version does it, when the code is building the bouquets, it simply checks for the existence of any data in Supplement.txt and propagates those channels in.
The core code of Enigma 2 basically ignores any numbering embedded in the bouquets and simply auto numbers the channels.
So if you look at the main bouquet, every time you see:
Code:
#SERVICE 1:832:d:0:0:0:0:0:0:0:
#DESCRIPTION
That's basically a dummy channel causing Enigma to increment the channel count by 1.
The numbering scheme is made up from the number of channels in the main bouquet (including all those dummy channels), so as long as the Setanta channels were added to the Main bouquet in position 423 & 424, they will turn up as planed just like the rest.
In the absence of support for Supplement.txt in ABM, I'm doing the same as you, manually dropping in the Setanta channels:
Code:
#SERVICE 1:0:1:c7a7:96c:2:11a0000:0:0:0:
#SERVICE 1:0:1:c7a8:96c:2:11a0000:0:0:0:
....but of-course auto scheduling ABM keeps over-writting them, so at the moment, I have backed off my schedule to 30 days so at least I only need to remember to add the channels back in once a month, hehe.