The fastest way to add existing items to a matrix is by using Power Ops.
If the item was created as a standard item and you'd like to associate it with a matrix, follow these steps.
1. Run Power Ops and locate the matrix class entry (M code). To view all current M codes press Ctrl-M
2. Right-click and select Add Existing Items.
3. Search for and locate the item(s) you'd like to add.
4. Highlight the entry and click Add to Matrix. If there are multiple items to add, tag them first.
5. The matrix grid view will appear listing all items in the matrix including the one you just added.
It doesn't matter if the new matrix addition has a different Item Lookup Code format than other matrix components, but you can change the ILC to match that pattern if you'd like.