You have a few options for this.
WORK OFF EMAIL NOTIFICATIONS TO ADJUST WHAT YOU WANT
With this method you can just add the items to skugrid but set the marketplace on the item to Do Not Reprice. You will still get email notifications and visually be able to see the item on the grid, but you would do price and stock changes yourself. This would be set per item that you want to function this way.
PAUSING INDIVIDUAL ITEMS
This method is similar to the first, but instead of setting the marketplace to do not reprice, you would leave that alone, but would pause the individual item. Again, you'd have to use email notifications and the grid to observe changes and adjust the listing for price and stock changes yourself.
SET A FIXED PRICE, BUT HAVE SKUGRID CONTROL STOCK PER ITEM
You can set a fixed selling price that you will have to change yourself is supplier price changes, but skugrid would still check and adjust for stock changes. To do this you would edit the item, click Override Selling Formula and instead of using the wizard, you could enter the price you want the item listed at...example 25.99. The item must not be paused and the marketplace still needs to be set. But in doing this, skugrid would keep the price at 25.99 until you choose to change it, but it would take the item in and out of stock when the supplier goes in and out of stock and based on your settings.
SET ALL ITEMS TO NOT CHANGE PRICE BUT ONLY CHANGE STOCK
With this method you would be telling skugrid not to change prices on all items, but to control stock on all items. To do this you would go to Settings --> eBay --> US or UK --> and change Auto Update eBay Stock to Yes and Auto Update eBay Price to No.