Are you referring to the keyboard shortcuts when creating an estimate? If so, just press Ctrl+Alt+E to open a new entry.
You can access the quick reference guide within QuickBooks Online. Simply hold Control + Option (Alt) + ? (for Mac). Then, you'll see your company ID and list of keyboard shortcuts.
The + New icon will also help you create any transaction you want.
To clarify, are you referring to the transaction icons in the Dashboard menu? If so, the option to customize the page to add extra icons is unavailable.
Meanwhile, you may want to utilize some keyboard shortcuts to speed up your navigation in QBO. For guidance, please see this article: Keyboard shortcuts for QuickBooks Online.