The IPM course in IIM Indore used to be a diploma course and earlier it was not possible to do a Ph.D. after graduating. But now because the five-year integrated IPM program is a degree course, you can easily enroll for Ph.D.
To the best of my knowledge, if you are a commerce student you are not eligible for the IPM entrance exam that is IPMAT. Commerce students with maths as a subject are still eligible for the paper. The IIM authority has clearly stated that maths has to be a compulsory subject in class 12th to be eligible for IPMAT. It is a primary criterion. You can also be eligible if maths is an optional subject instead of PE or CSC.