I want to inject a feature as a module in iOS. Could you please help me how to do that?

Not very clear.

1. I have an unknown bug-free iOS project.

Is it your project and do you have the code ?

What do you mean "unknown" ? If you don't know it, how can you define something for it ?

How can you say it is bug free (no software is ever bug free 😉)

2. I want to add a new feature.

3. Need to set up minimal dependancy to the actual project source.

Do you want to extend the source code ?

Minimal dependancy: do you mean you want to create a framework and use it in code ?

4. The feature should be reusable to another project.

That's the very purpose of frameworks

5. Will be safe switching between multiple modules.

????

This is up to your design quality.

Q1. Is it your project and do you have the code ? Ans: Yes

Q2. What do you mean "unknown" ? If you don't know it, how can you define something for it ? Ans: Because, It wasn't written by me from scratch.

Q3. How can you say it is bug free (no software is ever bug free 😉) Ans: True, But one exception, The Project wasn't reported any issues on it's long run.

Q4. Do you want to extend the source code ? Ans: Yes

Q5. Minimal dependancy: do you mean you want to create a framework and use it in code ? Ans: Yes

Q6. ???? Clarification: More I use the framework, More responsibility comes.... So, I was added an abstraction layer between each projects. It will be act as bridge between the framework & projects When the user need to update the framework will change not the bridge.