Insert a url segment without creating sections
From what I gather from your question you want some sort of sub-categorisation of your entries. 

Such that one entry can have the URL as:
    siteUrl/section/subject-1/entry-slug

while another entry can have a URL as:
    siteUrl/section/subject-2/entry-slug

If that is what you want to achieve then you have to look into Structures instead of Channels

Here is the link: http://buildwithcraft.com/docs/sections-and-entries#sections

You can basically have multiple entries with sub-urls in the same structure section.