To avoid confusion in the future, please open threads related to Sitefinity 3.x in their respective forums. The thread is opened in Sitefinity 4.x forums and this lead Zheni to the impression that you are using 4.x.
You need to create a custom web UI type editor which is going to be populated with tags. You need to use the respective manager to get tags list. Sample on how to create such controls can be found here: Creating a custom WebUITypeEditor
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items