To add text to an already existing text or spritefont object you can use the append action :
newline & your_text_to_add
This will jump a line at the end of the text object and add your text.
Now the issue is that you may have to resize the object and position it so that it displays only the new text.
So another way is to do the opposite :
Your_text_to_add & newline & Text.text
This way, the new line will always be on top, but on the bottom you won't be able to slide down to see the previous text (the object is not really meant to do that).
As mentioned if you really want to allow for a complete slide, you have to resize the object and then allow it to slide, using possibly some layer mask to keep it in a certain position.
Example of masking and scrolling