You can add and apply a custom override formula to all items from a specific supplier by following the steps outlined below.

1. Set your grid display to show 1000 items per page (that way you repeat your actions as few times as possible).

2. Use the 2nd filter on the grid and select the supplier from the drop down menu. (see video on using filters - If you've done step 1 and 2, then you will end up with up to 1000 the supplier's items on page 1 of the results (if you have that many).

3. Then, you will click Multi Item Actions --> Select All on Current Page (this will select all of the the supplier's items Items that are displayed on page 1 -- so up to 1000 of them if you've done step 1 & 2 properly). Here's a video on how to Select All on Current Page if you need a visual -

4. With the supplier's items selected, now you can just override those items and apply a new formula to just those items by clicking Multi Item Actions --> Add Formula Then you would apply the formula -- see video on Adding Formula to Selected items - This would create an override formula for just those selected items without touching the rest of your items.

5. Now it would be time to Force Reprice the Selected items -- which would be Multi Item Actions --> Force Reprice Selected (see video on force repricing selected items --

6. Note: If you have more than 1000 items from this specific supplier that you need to apply the new override formula to, then you would click on Page 2 at the bottom of your screen and repeat steps # 3, 4 & 5 for the next thousand items. Repeat as necessary until done. 

You'd have to give it some time to allow the force reprice to take effect, but if you click on edit on any of the supplier's items at this point, you'd see that they now have an override formula there...and that's what will be used to determine your selling price.