Yes, it is possible to use Modifier to make the Inventory Account a required field on the Purchasing Item Detail Entry window. The field can be set to required in Modifier, which only takes a minute.
However, this approach requires that you are licensed for Modifier & VBA (if you wish to do it yourself), or have a customization site license (if your partner creates a package file for you). You should check with your partner to see if you have either license.
If you don't have either of those licenses, and if you don't need any other customizations, and if cost is a significant concern, then another alternative is to have your partner or an ISV develop a Visual Studio Tools add-in that validates the field and makes it required. This is an unconventional approach and not something I would recommend as a first choice, but it is technically an option. A GP partner or ISV should be able to develop and deploy such an add-in in 1-2 hours.
A completely different approach would be to setup a Business Alert (Tools -> Setup -> System -> Business Alerts) to notify users if a PO line is missing the Inventory Account value.
And last, but perhaps most important, would be to change the focus from preventing a blank Inventory account on the PO lines to making sure that all of your inventory items have an Inventory account assigned to them. That way, the account can default from the item, rather than be entered (or not) at the PO line level. Run a SmartList on all of your inventory items to make sure all of them have account numbers assigned.
Hope this helps.