How to combine shape layers without precomp (copy paste content without changing position)?

The first thing you need to do is check your shape layer position. If the shape layer position has changed then the copied shapes will be out of position. As long as two shape layers are in the same position you can copy the contents from one shape layer and paste them in another. You cannot copy "Contents" but you can copy and paste groups or any other shape or animator that is under Contents. If you are going to have to adjust positions then it is a really good idea to select everything that is under Contents, right-click and choose Group Shapes (Ctrl/Cmnd + g), then copy and paste.

When you say you imported several shapes into After Effects it makes me wonder if you have imported an Illustrator document as a comp retaining layer size, then converted the vector files (AI Layers) to Shape layers. If that is your workflow, and you do not intend to extrude shapes using the C4D rendering engine or use Shape Layer animators like Trim Paths, or actually animate the vector paths, stroke or fill color, there is no reason at all to convert vector layers to shape layers. 

More details on your project would help. A screenshot of at least two of the shape layers the modified properties of both of them revealed (press uu) would give us a lot better idea of what you are facing.