Problems importing a big wordpress site into a wordpress network


I'm trying to import a wordpress site (approx. 20.000 articles) into a network.

My initial idea was to export / import the site. It faild because the export took to long and I can't increase the max_execution_time on the export server.

Second idea was to export different dates. This failed too because the attachments didn't export.

My third idea (witch I'm working on it now) is to do a backup of the initial site, install a clone on my server, then export (because on my server I can increase the max_execution_time) then import into my network.

As the backup, install, export and then import will take some time, I want to kindly ask if it's a good idea...