Make sure you have all available Unit of Measures (UOM) setup under the Inventory UOM Table for each item. Once the UOM table has been updated, go the Inventory / Vendors Tab and select the vendor you want to add. Once a Vendor is selected, you will see a UOM dropdown. Just select the appropriate purchase UOM.
When issuing products to your Departments / Patients, just use the UOM dropdown tab (under Usage Menu) to change the UOM before processing the transaction.