How to create a plugin for CB ?

Hello,
I'm interested to create a plugin for CB that allows cities choose in the regististration page.
Beforo to do that I would like know if:
- there is a plugin of this type yet;
- how to create a plugin for CB, because the article I found in this site is dated 2006 and for this not compatible with Jommla 1.5.x framework.

Thanks.

If you're looking for a single dropdown field with cities to choose from this solution article might be the easiest way to do it:
How to populate a CB Field with a Country List

There is a plugin to select country and state as a conditional field dropdown:
www.joomlapolis.com/cb-solutions/directory/community-building/profiles/cb-country-and-state-dropdown-fields

There is an incubator project for conditional fields (not yet listed on the website).