Well, this is a tough one, and I sympathize. There's no great way to do this, but the idea below might help.
First, you could add a 7th product as a none alternative. To do this you will need to go into the simulator and add a column for the none alternative. Through trial and error you can find the utility of the 7th product that leads it to have a 15%-20% share (whichever you want).
Now, this is a poor fix because it assumes, among other things, that the 7th product takes share proportionally from the other six, when in fact this may not be true at all.
Though I think this top-of-mind idea is a poor fix to your problem, it is a fix and I'll be curious to hear if other folks have better ideas.