You can use Set Expressions to format an output of your Gloop Service and use a Groovy script that will append the input string to whatever property you want it get appended. You can do this by double-clicking the output property you want to modify, and a Set Expression editor window will appear.

If you are not much of a mouse clicker, you can use ⌥+E shortcut (For Mac, alt+E for Windows and Linux) when selecting a Map Step, then use up/down arrow keys to select the property then press return(enter key) to open the Set Expression editor