If you want to do BBA then you have pass the 10+2 examination in order to be eligible for that. So with having a Diploma only, you are ineligible for the course of BBA. Now if you have completed your 10+2 then obviously you can go for it. Now you can go for the lateral engineering admission and complete your BTech. After that you can do MBA which is quite a known trend.

To know more about BBA visit the link below:

https://www.careers360.com/courses/bba-bachelor-of-business-administration

For career after Diploma in Engineering visit the link below:

https://engineering.careers360.com/articles/diploma-in-engineering

I hope this answer helps. All the very best for your future endeavors!