It is possible to package almost any DAZ Studio-compatible content so that it can be "installed" using DIM. But it must be properly packaged in order to do so. You cannot simply point DIM at unpackaged content that already exists on your hard drive. If you really want to do this, there is a way to do so, although I have no personal experience using them.
On the other hand, if you are wanting to simply have your custom content show up in "Smart Content" rather than just the Content Library tab, that can also be done by creating the necessary metadata file for it. That can be done regardless of whether the content is installed through DIM or not. For that, you can start here:
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It is possible to package almost any DAZ Studio-compatible content so that it can be "installed" using DIM. But it must be properly packaged in order to do so. You cannot simply point DIM at unpackaged content that already exists on your hard drive. If you really want to do this, there is a way to do so, although I have no personal experience using them.
https://www.sharecg.com/v/88851/browse/10/Software-and-Tools/Install-Manager-Package-Maker
which is free, or there is this product in the store:
https://www.daz3d.com/content-package-assist
On the other hand, if you are wanting to simply have your custom content show up in "Smart Content" rather than just the Content Library tab, that can also be done by creating the necessary metadata file for it. That can be done regardless of whether the content is installed through DIM or not. For that, you can start here:
http://docs.daz3d.com/doku.php/public/software/dazstudio/4/userguide/creating_content/packaging/tutorials/adding_metadata/start
There are also videos on YouTube, as well.