For example, if I want to add a bunch of lines in VS code using a single command and say I'm able to build this feature, can I use this on my version of VS code without the community acceptance?
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software [...].
Since you are asking about open source software in general, I'll add a more general part to my answer.
One of the elements of the Open Source Definition is the right to modify software:
3. Derived Works
The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
The four essential freedoms required by the Free Software Definition also include the modification of software:
- The freedom to run the program as you wish, for any purpose (freedom 0).
- The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
- The freedom to redistribute copies so you can help others (freedom 2).
- The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
If the license of a software prohibits you from making any modifications to it, then it is neither free nor open source!
The freedom to modify a piece of software is an essential open-source freedom covered by any open-source license, specifically the MIT License which VSCode is licensed under.
In other words - you most certainly can do this.