To add one or more Sub Assets to an existing asset, simply select the ones you want to add and drag them on top of another asset.
Dragging SecondaryMission_1 on top of MainObjective
You will be presented with a popup:
Add
By choosing to just Add, the dragged asset will be added as a sub-asset to the destination and disappear from its original location.
SecondaryMission_1 is now a sub-asset
Add and Keep Original
By choosing this the original asset will stay in its place, and a new copy will be added to the destination. In this case, all references to the original asset will be preserved.
SecondaryMission_1 is now a sub-asset,
but the original is also preserved
It's not possible to add sub-assets to assets that already feature sub-assets as a result of the import process.
For instance, adding sub-assets to an .fbx model will result in nothing, because as soon as the asset saves, the import process will reset the sub-assets to the default ones.
See a list of the types that can't be added as sub-assets on the Exceptions page.
Create a new sub-asset
It is possible to create a new asset directly as a sub-asset. Simply right click on the destination main asset, and use the Create menu:
Using the Create menu while an asset is selected, to create a new Material
Once you select the asset type, the new asset is created and you can give it a name, like usual:
Renaming the new asset
Once the rename is confirmed, the new asset will be added as a sub-asset to the previous selection.
The new Material is now a sub-asset
Preserving References
When adding a sub-asset using drag and drop, normally the references to the original asset will break. SubAssets Toolbox has a way to prevent this, and you can find more about it in Patch References.
Renaming sub-assets
To rename a sub-asset, select it and use right-click > Rename Sub-Asset. You can also select it and hit Enter (on Mac) or F2 (on Windows).