I have two Universal aluminum self supporting towers. The 80 footer has a 30" triangle base. The 70 footer has a 26" triangle base. As I remember they make bottom sections in 30", 26" and maybe 22" triangle spacings with either "straight" or "tapered" ends.
Assuming you have the Universal tilt over (hinged base) yes you can add one or more sections at the bottom as long as the triangle dimensions of the new section(s) are the same as those of your base and the existing bottom section.
Doing so may (will) alter the windloading specs. Call Universal. I have found them to be very helpful in the past.
Maybe I misinterpreted the question but he said "removing the base stubs," which are the long steel posts that set in the concrete to anchor the tower...? Once they're in place, I can't imagine removing them.
If he just wants to add one straight sided base section, those stubs would never have to be removed; he'd just unbolt the existing base section and bolt the new one in place, to the folding brackets on the existing stubs.