I found a solution to this:
A year ago, I activated a sponsorship with a duration of 12 months. However, we used the credit in around 10 months, so I wanted to apply the second sponsorship to this subscription right after we used the funds. When going through the activation process, a new subscription was created and I wasn't able to apply the sponsorship to the old subscription. However, I stopped the process before activating the sponsorship on this new subscription and waited two months, until the first sponsorship ended (the 12 month period). Once this first sponsorship was over, I was asked which subscription I would like to add my new sponsorship to - here I was able to pick the old subscription. Subsequently, I deleted the newly created subscription as we don't need it.
For us, this means the subscription was in mode "pay-as-you-go" for two months but at the same time we didn't have to spend time on deploying new resources on the new subscription.