Just in case anyone else wants to assign a username or dummy email addresses at the end of a survey (maybe for use in future external survey or application), I found a far easier workaround than having to deal with the database... outlined below.
In my case I needed to assign a username/email address to participants of my public survey (without registration) which they can use to sign-into other survey software (a mobile application for an experience sampling survey which requires registration), without a lag-time of me doing it manually with their actual email address.
As LimeSurvey automatically assigns a predictable ID to each response, it's as simple as utilising this ID. As mentioned on the following page (
www.limesurvey.org/manual/URL_fields#Basic_fields
) inserting this code in your survey {SAVEDID} will input the LimeSurvey response ID. This number is allocated sequentially for each response. I inserted the ID with some additional text, in order to use it as a dummy email address:{SAVEDID}ime@example.com, with a static password: 54321. This information was inserted into the End Message and also included in the email confirmation for future reference.
A nice tip that I found for creating dummy email addresses for import into the other application (particularly in a sequential manner such as in this case) is to use the following formula in Excel: =ROW()&"ime@example.com"
Hopefully that is helpful to someone!