Hi @pnagendra081, combining subassemblies in SAC is not recommended. One of the main benefits of creating subassemblies is that they are the broken down geometry of a more complex cross section.
On the other hand, what you aim to do is a different thing, but it is possible. To save previously created Assemblies on your palette and have them ready to use follow these steps.
1. Open the file that has that Assembly you want to save. Have just this file open, close any other file or Active Civil 3D window.
2. Open the Tool Palettes
3. Create a new palette. Assign the name of your preference.

4. Select the Assembly Baseline Marker ( As you see in the following image). Just the marker, not the whole Assembly with the subassemblies (just that little symbol in the middle) and keep your left click pressed. Drag and drop this to the new palette you just created.

5. It should look like this:

Now you can use your assembly in different drawings.
You can do this with either out of the box subassemblies or Custom Subassemblies. I hope this helps.
Best regards!
Camilo Fernández
P.Eng | Transportation
