is it possible to call a DBTest from Extension tool??
What do you mean by "DBTest"? Do you really mean SOAtest's "DB Tool" or something else? What is it that you are ultimately trying to accomplish using an Extension tool? Are you trying to script part of a SQL query or something else?
I am using extension tool validate JSON response against the Database. So i have to call multiple times, thought it would be better to call and get the response in code itself.
You should use the DB Tool to execute a SQL query. The SQL result set can be validated by chaining a validation tool to the DB Tool which could be a Diff tool, XML Assertor, or an Extension tool if you really need to script your validation.
As far as how to run things multiple times, whether it be a DB Tool or an Extension Tool or something else, there are generally two approaches. One is to drive the test execution with a data source where the test or scenario will run multiple times depending on the number of data source rows. The other is to use test flow logic, where you configure the test suite to loop based on the value of a test suite variable.