Yes, why not they Can purchase but can’t get ITC on that purchase. A Composition dealer under GST can purchase from other states and has to pay only 1% of sales to the government. But, he can’t claim the Input Tax Credit (ITC). Also, outward supply is restricted to intra-states only.
But if a person wants to switch from a normal Registration to the Composition Scheme, he will have to file an intimation in form GST CMP – 2 and form ITC – 03 with complete details of the ITC related to Inputs, semi-finished and finished goods held in stock.