What is the end goal of making it a single model? If you want to be able to move it all as a single object, you can already do this by selecting the models you want to move together and then going to Edit > Group Models or CTRL + G. This will allow you to resize, move, and rotate as a group.
Here's more on grouping: https://www.simplify3d.com/support/vide ... ng-models/
Please let me know if this isn't the functionality you were requesting, as I'd love to better understand in that case.
"A bird does not sing because it has an answer. It sings because it has a song."