Do you mean they appear at the bottom when you drag an action onto a project in the sidebar (e.g. dragging new actions from your Inbox to a project)?
If that's the case, then no, I don't think there's a way to control the new action's position. It always appears at the bottom of the list.
On the other hand, if you're in a project and want to add a new action to the top of the list, you can select the current top-most action and hit Shift-Return to create a new action above the existing one.
You can also select a list of actions (or hit Command-A to select all in the current projects) and choose Edit -> Sort -> By Date Added to sort the items by creation date (in descending order with most recent additions on the top).
Hope this helps.