You could create a 64mb partition and do a frugal install there.
Then you could create a larger partition (they recommend at least 2gb) and do a full debian install there. I
presume that you could save the mydsl folder and the backup.tar.gz file for your frugal install on the second partition. I believe you also need a third, swap partition (set at about 2 times your RAM) for the full Debian.
In your boot loader (I use grub) you'd have to create an option to boot from frugal and a second option to boot from the full Debian.
I have to ask though, if you're installing a full version of Debian anyway, why not forego using DSL and just install a full version of Debian? I do not see the benefit of using the "DSL" version of Debian when you could just use the regular Debian installer to do a minimal install on that second partition.
(Now, personally I only use the frugal install. The benefit being that if I ever screw something up royally, all I have to do it reboot without backing up and I get back to my previous state. I tend to screw things up royally with disturbing frequency.
The other benefit is that I can always choose at boot-time to not load my settings and/or my MyDSL extensions, so I get the default DSL configuration. This is sometimes useful if the modifications I've made to my own setup are getting in the way of something I need to do with the computer. The third benefit is that when a new version of DSL comes out I simply replace the
KNOPPIX file on my harddrive with the new one, and it doesn't make any changes to my saved settings.)