If you build your own Slicer, you are also expected to build all extensions, to ensure ABI compatibility.
Once you have built Slicer, it is easy to build all extensions, since their build is fully automated. You can bundle any extension in your custom build so that your users don’t need to install any extensions.
You can also try to replicate build environment of official Slicer builds on your computer (so that your custom Slicer build is ABI compatible with extensions in the official app store) force your repository revision (by default a git hash) match a Slicer stable or nightly release revision (still SVN revision numbers) and then your extension manager will find the corresponding extensions.