Any method to calculate constant operations?

There is a lot of codes using our old platform need to be adapted to our new platform. So, I have to make out a plugin to help developers to simplify this work.

There is a requirement that make the annotations on method from the old style to the new style:
OLD:
@Location("/sys/getUser")
NEW:
@Uri("cmbus:///sys/getUser")
This is quite easy. I just scan the method and add new annotations on those method.
The old value can be achieved by calling PsiLiteralExpression.getValue().
The perfix-value is added by:
 

private PsiExpression getExpressionFromText(String value, PsiAnnotation annotation) {
    return JavaPsiFacade.getInstance(project).getElementFactory().createExpressionFromText("\"" + PREFIX + value + "\"",
            annotation);
}

But there are some cases making the situation quite difficult. E.g.:
OLD:
private static final String BASE_LOCATION = "/mgr/mer"; // in some other class
@Location(BASE_LOCATION+"/addCer")
NEW:
@Uri("cmbus://"+BASE_LOCATION+"/addCer")


So, I can't get the value directly, and don't know how to generate the PsiExpression in these situations.

Is there any method to calculate these constants, so I can put the target values into the annotations easily?