Bulk import proposal (content + attachement) + users

In our case, we have an active community for more than 3 years. We have more than 2'500 active users and no less than 5'000 ideas, proposals, answers that have been produced during the last 40 months.

However, it is currently not possible to take over a complete structure. The only thing I can do is to re-import the proposals made, but they will all appear under my name.

Would it be possible to link the users as well so that I don't have to start from scratch? The idea would be that the content can be taken with all its layout because sometimes a proposal is more than 1 page A4 of content (bold, bullets, titles, table, ...)

This is currently the most blocking point for us.