What if it takes you a couple of years to do your post-bacc? Can you list some undergrad EC's even though you may have been out of undergrad for five years or more by the time you apply to school?
Hopefully you'll have plenty more recent ECs to fill up the slots, but if you think some of your undergrad ECs were particularly relevant/good/interesting, you could list them--they don't "expire." However, I wouldn't list too many of them, or it'll seem like you haven't been doing anything worth mentioning in the past 5 years. Another option is to lump your undergrad ECs thematically (like "leadership" or "sports") in one or two slots, and list multiple items that belong in each category. That way, you can show that you did things other than study in college, but you won't look like you don't have any more recent ECs to report. Good luck!