Hi

Yes you can do that. Let me clear the things here. 

Firstly if you register for CSAB Counselling and get a seat through it, you are bound to accept the seat. And the last round to withdraw from JoSAA Counselling 2019 is also the 6th round. In the 7th round, you simply cannot withdraw any seat in JoSAA Counselling 2019. So if you're sure enough to get seat through CSAB, consider withdrawing the seat from JoSAA in the 6th round, because if you do not do so, your paid amount will not be refunded. 

Best of Luck. 

Hope this helps.

Cheers!