Unfortunately the answer is no, not in the way I think you mean. You cannot enter a single book record which could have multiple series index numbers associated to it.
Having said that, yes you can fudge things a bit and get some equivalent. You could enter the book record multiple times (ie, have three copies of the omnibus record each with it's own index value, but with book formats only for the first copy). Or enter the omnibus once, numbering it correctly for your personal collection only (so in a 5 book collection, with the omnibus including books 1-3, you would have omnibus , book 4 is , etc). You could use the fractional portion of the index to represent the book number range (so, as above, omnibus becomes [1.13], book 2 is , etc).
I am sure their are a bunch more. I haven't really dealt with any omnibuses as yet. I've got them, I just have them kind of sitting off on the side though until I can get other parts of my database better organized.