Hi Diptorup

Yes, you can take admission in B.Com or B.Sc. in IGNOU itself. Note, that with even with BPP, you will not be able to take admission in graduation programmes of any other university. You may take admission in any graduation programme offered at IGNOU. After that, you may use your graduation degree anywhere to get admission in post graduation.

Bachelor's Preparatory Programme (BPP) of IGNOU is a six-month preparatory programme offered for those students who wish to pursue higher education (graduation and post-graduation) but have completed schooling only till class 10th and haven't done class 12th.

Hope it answers you.
All the best!