While what you prpose is not the norm, I can't see how a background in business would put you at a competitive disadvantage when applying to grad school in architecture. I think the schools will look more closely at your academic record and your board scores than they will the undergrad discipline. But, I could be wrong about that.

I did the opposite - B. Arch as undergrad / MBA later. The combination of degrees is, IMO, very strong when it comes to building a career.

What I found interesting in grad school is the similarity between the thought process of the two disciplines -- both require creativity and neither is based on a calculated right answer -- there are a infinite array of possible successful solutions to any architectural or business problem.

I got a BS in Management and Design in undergrad and then got my MArch. See if you can take some courses, or rework your degree to get some experience in your University's art department. Takes basics like 2D and 3D composition, some graphic design classes to learn photoshop and illustrator, and see if there are any classes teaching 3D modeling. You can learn Autocad on your own. I wouldn't waste time taking a class to learn it. Maybe even take sculpting, ceramics, or textiles to learn about materials and how to make things with your hands.

The work you do in those classes could be used in a portfolio, but you'll also want to spend some time on here reading about portfolio development. You would probably even want to create some projects on your own and work critically to achieve a level of evidence that would demonstrate to a school you would be a good fit for Architecture.

Unfortunately, I didn't take the right classes in undergrad in the Art Department. It mainly had to do with ignorance on my part as well as a faculty that didn't really know how to prepare someone for a competitive Arch school. I have been successful in the long run, but if I could go back, I would have focused heavily on the types of art/design courses I mentioned above and I think I would have been even better prepared for Architecture school.

If you school has pre-Architecture, be careful about that. Some schools, for some reason, have set up their program to teach more about construction science and drafting, which really isn't a primary focus of Architecture BArch or MArch degrees. I am not saying it isn't covered or important, but I have noticed that some pre-Architecture programs at Liberal Art schools have somehow missed the point. They aren't really teaching students to think critically, but are just training them to be cad drafters.