Hi Kumar,
I'm not sure whether I understand your requirement, but I share my verification.
This is a text file. C:\work\a.txt
Created 2 actions using [Add Text to a File] action type.
First one is to add "aaa" after "111" with linefeed.
Second one is to add "bbb" after "aaa" in same file.
Set actions to a process like following, and run.
And then, insert "aaa" and "bbb" to the file.
I hope you can clarify regarding this action type.
If this info is not enough for you, please share the detail of your requirement.
Thanks
Yas