How can I do this??

Please tell Me if you know

To tell the truth I don't know what you excatly mean.

Well there are three possible answers I can think of. The first one is to join two selected meshes to one with Strg+J  in object mode this will "combine two meshes into one"

The other one is to work with shape key . I'm not exactly  an expert in it, but the basic idea is to transform on form from a mesh to another.

Or do you mean to copy one mesh from a blend file into a other blend file? That's pretty easy. Select the mesh you want to copy, press Strg+C, go to the blend file you want to past it and press Strg+V in the Viewport. 

 well you can apply sticky notes to objects like that applying mesh to another mesh so it will stick to it

Hi shanr, maybe use snapping to place the mesh. then select the mesh, then SHIFT select the other mesh you want it to stick to.

Press CTRL+P to 'Parent' it:

Now it sticks.

Thanks spikeyxxx  You are very helpful. Stay safe at home everybody!