Our design algorithms use goals in coming up with what concepts to show. One of these goals is that levels should be used an equal number of times. What you want to happen goes against this goal as it will oversample the status quo level. Unfortunately there isn't an easy way in the software to do this. However, I can think of two approaches:
1) In general I think the only way to accommodate it would be to put in prohibitions to prevent the combinations you don't want. This would be lots of cross-concept prohibitions (don't allow A/B/C, A/C/D, etc.), and would require the Advanced Design Module.
2) However, because it is an alternative-specific setup (meaning it sounds like no other attributes apply to the status quo level), then you could probably generate a design with the original 4 levels and the design set to only show 3 concepts. Export that design to Excel, and the modify it by hand to add a fourth concept to each task that is level 5, and then zeroes for all the other attributes. Add level 5 back in to your design and then import your modified design.
Make sure to use the Test Design routine to ensure that this deviation from efficiency isn't causing too much loss of information from your other levels.