Hi Im super new to Zbrush. Im not sure if this is the right platform to be posting helps but I am wondering how does Dynamesh function enable me to fill up this particular shell. Are there any other options to fill up the shell using other command, etc? Many thanks!
Welcome to ZBrushCentral @Chew_Cheng
This is exactly the right place to ask about questions. I am not 100% what you are asking but I would like to send you these useful docs for a basic understanding how dynamesh functions. In ZBrush 2019 you could also use sculptris pro function and there is a new function which allows you to merge objects. (Go to Tranpose Mode --> Wheel Icon - remesh by union) Basically it allows you to remesh where the meshes intersect, basically like a boolean function but its very effienct.
Here is some more information about Dynamesh:
http://docs.pixologic.com/user-guide/3d-modeling/modeling-basics/creating-meshes/remeshing/
I hope these documentaries find your question and are useful for your project.
If you havent seen these free tutorials created by many talented artists, There are many online ressources, I would recommend you to check out some of those websites to see if there are any quick tips which are useful for your creation
Happy sculpting! 
Kenny
Hello! Thanks for the reply! Wow Zclassroom is great! Thanks for these links!