hi there Viktoriawagner80 !
You can "Combine" the meshes you want from the "Objects" menu, there is a button with two arrows pointing to each other, then choose the meshes to combine and click on the two arrows again to turn them into 1 mesh.
If you then want for those to be "solidified" you can run a "Remesh" from the Mesh menu. (I'm planning on adding a shortcut for this in the future, so there is a choice for regular combination or remeshing-based)
