In my approval workflow, I have a custom Submit for Approval button. Is there a way to incorporate it with Save, so that users can click Save and Submit for Approval using a single button when needed? Some users do not want to click 2 buttons when they need to save and submit at the same time.

That is possible. My company has done it before. We made it so that the save button also triggers the transaction going to the next step for approval. We added a message on the screen saying their transaction has been successfully saved and submitted for approval.

Just to clarify, what I prefer is to have a Save button and a Save and Submit button separately. So that the users can choose.