@yukinoh1989 You can’t merge objects while sculpting. In the beginning I opened Blender 2.79b. In blender delete everything in the scene. Then I hit Shift-F1 to open the Append dialog window.

In blender 2.79b make sure you have activated the Bool Tool – addon in the User Preference Dialog,
Select 2 or more objects you want to join and click the ‘Union’-button on the Toolshelf, under ‘Bool Tools’ (probably at the bottom).

Then give the new combined object a meaningful name (makes it easier to import the objects back to blender 2.8) by double clicking on the name in the Outliner. (CTRL-leftclick on the name works too.)

When you’re done, save the file under a new name (for example temp.blend).
In blender 2.8 hide or delete the objects you just combined in 2.79b. And then append the new object(s) from the temp.blend file you just made, the same way you did in blender 2.79b, but to get to the Append window, you press F2, and then click append.