add extension to ringgroup programmatically

Hi folks,

i was wondering if there is a way to add an existing extension to a ringgroup?
i already tried with updating the string in the mysql database:
mysql> update ringgroups set grplist='**ring groups go here**'where grpnum=**grpnum goes here**;
with a following reload of asterisk.
this didnt work, the device logged in to the extension doesnt ring at all, so i think theres another place where i have to make some changes to take effect?

any help is very welcome
thank you for your replies

regards
Johannes from Germany