I have done my B.Sc in mathematics. Can I pursue B.Tech?

No, if you are done your B.Sc, then it is very difficult to pursue B.Tech in any field. Rather, it will be wastage of time, money and efforts. So, you should better go for M.Sc/M.Tech( Statistics) .It will be beneficial for you and your domain of Mathematics will sustain. So, all the best for future

Yes, you can pursue B.Tech. You have to clear the required exam or whatever the required eligibility is of the particular institute you want to take admission in. There is something called lateral entry scheme in which you can get admission in directly second or third year.