Can I append text to the title or the description of a card?

You can  insert or append text to the name of a card by renaming it and using the {cardname} variable:

when a card is moved to list "Done" by anyone,
rename the card to "{cardname} - DONE!"

The variable {cardname} will be replaced with the current name of the card. The end result will be the current name of the card plus the additional text. E.g. a card titled "Task" will be renamed to "Task - DONE!"

Similarly, you can extend the description of a card:

when custom field "Customer ID" is set by anyone,
set description to "{carddescription} \n\n Customer ID for this order: ."

Notice the use of the special sequence \n\n. This introduces two new line characters, which will break the text into a separate line in Trello. Setting the description of a card with Butler uses the same markdown format as doing it through the Trello user interface.

Also notice that more than one variable can be used. The example shows the use of a Custom Field variable to append the value of a custom field.