I think you want an extension ticket, but see below.
Short Trip Tickets requiring validation must be validated on starting travel.
The idea is that you need to have one ticket that's valid for your whole trip.
Technically, on the journey out, you may step off the train at the last zone B station, validate the ticket, and step back onto the same train. But unless you know exactly where the validator is on the platform and position yourself accordingly, you'll be stranded until the next train comes.
On the return journey, you need to validate your single ticket at the start of the journey, and you're supposed to step off and back on at the first zone B station. Technically, if a ticket inspector happens to step on at that point and realizes you did not have a ticket for the whole journey, the rules state that they should fine you.
There is a different ticket, the extension ticket (Anschlussfahrschein), which is meant for use in combination with an ordinary A+B ticket to prolong one trip into zone C (or start in zone C and continue through zone B and optionally A). This ticket is explicitly not allowed in combination with the A+B WelcomeCard or with the A+B CityTourCard. I can't find a definitive reference regarding its validity in combination with a daily, weekly or longer ticket. The official FAQ only explicitly excludes “WelcomeCards, CityTourCards, Schulklassen-Tickets und FamilienPass-Monatskarten”, so I surmise that it is valid in combination with other passes, but please confirm with a ticket office first (and report back!).