What Distro are you using?
with SuSe you don't have to give the root password, just give kppp suid root with chmod +s KDEDIR/bin/kppp and i can use kppp as user.
When does kppp ask voor a password?
If it doesn't when invoking kppp in Konsole, then probably the kmenu-entry of kppp contains a command like kdesu -c kppp . Remove the kdesu-option part to start kppp without root password.
Also make sure that kppp gets the suid root to get the right acces permissions for your modem device.
(this is considered as a security breach, that is why kde does not set suid root by default, and apperently some distro's choos to use kdesu in stead.
You can also create a seperate group for internetconnections, make kppp (and pppd) member of this group, and allow this group to acces your modem and the proper files. - for more details see kppp->help)
Rinse