I am trying to read a .xls file using a spreadsheet file node and use this data as an input for another node, but when trying to select the values from the JSON to add as expression I am only getting the first entry as a value, all the other data is lost.
Is there any way to get all the data as an input?
The data is not lost. It does only not get displayed there. The Expression Editor simply always just displays the data of the first item. When the node executes it uses the data of all items.
So if you for example plug that data into a Slack Node it would send multiple Slack messages. The first message would use exactly the data you can see in the Expression Editor. The other messages will then use the data of the other items.
You can an example for that here: