Well... I knew, it wouldn't be possible. It was worth asking anyway.
I'll just stick with what I have, unless you wish to do a brstm which includes the extended part and loops as normal (not forgetting that introduction piece from MK: DD!)
There's one other track that I know of that has an extended part is the ending from Super Mario Bros.: The Lost Levels.
To be honest... I thought the Multi-Channel BRSTMs were big enough. The more channels a BRSTM has, the bigger the file size is and that also depends how long a track is as well.