Johan,
You could make use of a workaround for this: make use of an unused field and set that to "text to go before {caption}".
Apply this setting and then go back and set your caption field to "{variable for unused field above}" and apply this. Now your caption should be what you want.
Note: Take care to use a field that has no restriction on the number of characters you put in it.
Hope this helps,
Hayo