There's already a merge topic function under Moderation Actions, assuming I'm understanding you correctly. If you literally mean only the first post in a topic, what are you trying to accomplish and what would happen to the rest of the posts in the source topic?
I want to merge, for example, the first and second post in the same topic.
Merge the first post in the topic
in Feedback
Posted · Edited by Ramzzes
I want to merge, for example, the first and second post in the same topic.
In IPB 3 was such a function.