To append text to tags see this FAQ entry.
Action type: Format value
Field: TITLE
Format string: %title% yourcustomtext
For the actual text you want to append, is it in an existing tag or just something in your head at the time? If the former then you could format the title using the custom tag itself, if the latter then instead of creating a saved Action use 'Actions (quick)' for one-time use per album.
For changing the custom text in the titles use the Replace entry in the Actions drop-down selection:
Action type: Replace
Field: TITLE
Original: [Tokyo Love]
Replace with: [Live: Tokyo]
I tag all my collection with custom tags, such as ReleaseType, Live, etc, which makes filtering in programs such as foobar2000 much easier. Adding custom tags would make this process a lot simpler in the future for changes and automation.