Is there any way to migrate existing askbot based website to Discourse ?
There is an importer: discourse/askbot.rb at master · discourse/discourse · GitHub. It was updated just under a year ago. It probably worked for someone then.
Whether it’s “easy” is left as an exercise to the reader. It’s likely “easy” if you pre-pay for a year of Business hosting. Otherwise, it’s “easy” if you’re comfortable with a command line and can adopt the #howto for some other import script (most of them operate very similarly and this one looks typical except for expecting the database to be in Postgres, which is easier in some ways).
If “easy” doesn’t mean getting your hands dirty with some code and a command line then you can post in #marketplace.