I highly doubt it. Pretty much every course at UCL is oversubscribed, so I don't think they'd allow you to pick up another language when you arrived. The faculties tend to look down on people who get to uni and then decide they want to do something else. They're harsh enough on people who want to drop half their JH course to focus on a subject they've experience of studying, nevermind someone seemingly randomly deciding to take up a new language on impulse.
You'd have a slightly better chance of switching from German to Mod Langs if you emailed them between receiving your offer, and results day. However, Single Honours are a lot less competitive than Modern Languages, so just because you're accepted for Single German, doesn't mean they'd think you suitable for Joint Honours.
Your best bet is to give some real thought before you apply. You'd have a hard job convincing UCL that you really want to study an ab initio language as part of your degree at a later date if you did originally apply for Single Hons. The choice of ab initio modern languages is also fairly limited: Italian, Russian, Scandinavian and East European languages are all I can think of from the top of my head. People manage to write personal statements when they're applying for more than one subject, so I'm sure you could manage the difference between sing Hons/mod langs.
You could always take classes on the outside of your course at the language centre, but you'd most likely have to pay to do so, and they wouldn't count towards your degree classification.