Hi,

YES, you can join automobile design course after completing diploma course in automobile engineering department.

After diploma, you can go with degree level courses or PG diploma programs. B.Tech degree can be obtained with specialization in Automobile Engineering or Automobile design Engineering. Admission to the course can be done through lateral entry and thereby you can save one year. Post Graduate Diploma in Automobile Engineering is also suitable for you. Or else you can try for a job with railways, defense, government departments etc.

As you are already an engineering undergraduate.
You can opt for any one of the following courses for your PG:

1. Transportation & Automobile Design at National Institute of Design, Ahmedabad.
The eligiblity criteria for the aforesaid course is:
Degree in Industrial Design, Engineering, Technology, Architecture or Interior Design, OR Diploma in Design (under 10+2+4yr).

2. M.Des in Mobility and Vehicle Design at Industrial Design Centre, IIT Bombay.

3. M.Des in Industrial Design at IIT, Delhi.

4. M.Des in Industrial Design at IIT, Kanpur.

5. PG: 3 years certification program in Transportation Design at DSK International School of Design, Pune.

6. 2 year Post Graduate Diploma Programme in Automobile Styling at DYP-DC Center for Automotive Research and Studies, Pune.

Hope this helps

All the best.