When you enter the tracklist for the release, you can set the track numbers as 1.1, 1.2 and 1.3, for example, and then set the artists credits per track index. That’ll likely wreak havoc on people trying to use this data for tagging, but tagging isn’t MusicBrainz’ focus. 
I’m leaning on a precedent here in the forum (that went into two releases, and the way these track indices were entered in 2016 is still in the releases’ tracklists): MB Community: “Subtracks? (Classical) (edit: Track indexes)”
The two releases with track indices added to MB as a result of that forum post are:
You have selected an interesting release to add, indeed, but perhaps for different reasons! (I've hidden this text because it isn't central to answering your question. Click to show more.)
I’m not sure from where the Discogs editors got the information which performers performed on tracks 1.1, 1.2 and 1.3: the CD booklet lists all these four people as performing track 1, the vinyl releases’ liner notes don’t provide performer listings at all…
Track 8 has no performer information whatsoever in the CD booklet, and the vinyl releases’ credits on discogs don’t list who recorded this track.
And track 9’s credits are given as “the same artists plus [four more performers]”… What!? Who are these same artists? All the performers from Sanikiluaq, because track 9 was recorded in Sanikiluaq according to Discogs’ artist credits for the vinyl releases? Not sure about that… 