To add customized Html content to the Toolpart panel, we need develop a Toolpart object, attach it to the webpart object.
the steps to add a toolpart object to a webpart:
1. Create a ToolPart object implement the Microsoft.SharePoint.WebPartPages.ToolPart
2. override the ApplyChanges method, save the values entered in the ToolPart panel to the webpart, calling this.ParentToolPane.SelectedWebPart can get the reference object to its webpart object
3. override the GetToolParts method in the webpart class, add the customized ToolPart object to the ToolPart list
Note: to retain the properties value defined in the webpart, the properties must be set to Personalizable attribute. PeralizationScope.Shared will make the variable visible to everybody, PeralizationScope.User will make the variable consistance and retained for the specific user.