What is the easiest way to embed a customized 3D editor with final package?

Hi. I’m pretty newbie in UE4. I wanna to create a moddable game that user can select and modify features of objects. What is the easiest way to embed a customized 3D editor with final package?
For import and export of 3d models(FBX or obj models) can i use UE4 importer(Exporter) directly in final package? Is it legal?

I’m a legal guy rather than a technical one, so I can only answer the EULA questions. Under the EULA, you can’t distribute a game that contains Engine Tools to the general public. Engine Tools means:

(a) editors and other tools included in the Engine Code; (b) any code and modules in either the Developer or Editor folders, including in object code format, whether statically or dynamically linked; and (c) other software that may be used to develop standalone products based on the Licensed Technology.

It sounds like what you intend falls into one of those categories. In that case, you can distribute the game however you like but you can only distribute the Engine Tools to other UE4 licensees. That can be done through GitHub or potentially through our Marketplace (see the Ark Survival Evolved game as an example).