Inventory adjustment and enter zero (0) as the cost.

Once I am in inventory adjustment, it wont allow me to change the unit cost.

Use Receipt Type transaction instead of Adjustment.

Yes to use of the Receipt vs "adjustment" .  Since they are are zero cost you won't have an issue with the default GL accounts.  If they had a cost, you would need to be cognizant of which accounts are being updated with receipts versus adjustments.

It depends on your item valuation as to whether they will be in Sage at cost. 

If you have the PO module, do a receipt of goods (with invoice #) at zero cost.  Some valuations (with cost tiers, FIFO, lot, serial...) will post the inventory with zero cost, but others (average / standard) will post inventory with cost, and a purchase variance.

If you don't have PO, do the IM receipt, possibly making a GL correction afterwards (for average / standard cost items... purchases clearing to purchase variance).