The short answer is no.

Theoretically it is possible, because you could merge all of the records together into a single database, and they would all work.

In practice you have to maintain the relational integrity of all the users, tabs, modules, etc etc together. And because each separate install starts from zero, you're going ot end up with multiple portal Id's of 1, tab Id's of 67, module ids of 203, etc.

It will be far easier to recreate each website from scratch as a new portal and copy/paste the content from the old sites into the new sites. You might also be able to use the export/import functionality that some modules support.

If you have sites with user memberships you'll also have to find a way of exporting/importing users and roles as well. There may be existing tools around for this, I'm not familiar with them.

DNN Installs are a bit like humpty dumpty. It's quite easy to split them into separate pieces, but very hard to put them back them together again. The same goes for any relational database system, really.