Yep, I called back and there was no problem. I have no idea why the first agent wasn't able to help me.
Being new to the world of Continental, I found it interesting that because I had the requisite miles to make the reservation, I could NOT place a 3-day hold, whereas if I didn't have enough miles, I could place a hold.
I like to place a hold wherever possible (in case any changes pop up etc), so I will simply transfer some miles back to my UA account, make a reservation, place it on hold, and then shift the miles back when I want to confirm the reservation.
IIRC correctly, UA allows 3-day holds regardless. Any idea why CO does not?