Hey Nikita,
No, you cant do 12th while doing diploma, as it is not allowed in India and it is  even not suggested as diploma is equivalent to 12th studies.
If you are doing diploma in Pharmacy from any recognized university then you can directly take admission into Bachelor in Pharmacy (B.Pharma) course through lateral entry, there is no need to do 12th for this.
You are perfectly eligible for any Bachelor's degree after doing diploma in that field.
Hope you get it and don't worry about doing 12th now (if you are doing diploma and surely wanting to enter in the same course).
All the very Best!