how to do inventory adjustments without GL impact?

If it increases your inventory, you can assign a 0 cost. Effectively, no GL impact. If it reduces your inventory, I highly doubt it's possible to remove the GL impact unless the inventory value of the on hand is currently at 0 which is unlikely and absurd. You can create a JE to reverse the GL impact though. However, why would you do that? Is there a valid reason?

for our netsuite account, we imported all our journal entries since beginning (5-yr old company). Hence, inventory adjustments will duplicate prior entries in inventories.

Same approach as open AR/AP should work here. Post the CR offset of the inventory adjustment to a wedge account and then post a JE that DR that account and CR inventory to address the double entry from your historical JE.